Manpages

NAME

FBB::DateTime - Performs Date and Time Computations

SYNOPSIS

#include <bobcat/datetime>
Linking option: -lbobcat

DESCRIPTION

FBB::DateTime objects contain and represent date and time values. Individual date and time fields can be requested or modified, returning ’sanitized’ times (e.g., a date like March 33 or a time like 56 hours are never returned; instead the next month or day is returned). Times may be specified in local time (according to the computer’s idea of what the local time is), in Universal Time Coordinated (UTC) time values, as a time in a named timezone, or as a time in a user-defined timezone. Refer to the section TIMEZONES below for a detailed description of how timezones can be specified.

Dates/times represented by DateTime objects may be modified by adding, subtracting, or setting (std::chrono) seconds, minutes, or hours, or by specifying (a selection of) fields of tm structs.

Date/time modificationsthese are always performed relative to the DateTime object’s current timezone (which may be UTC, local or another timezone). Conversions between timezones (including the UTC ’timezone’) are also supported.

DateTime objects can be constructed in many ways. Dates and times may be specified as UTC time, as local time, as (std::chrono::minutes) shifts with respect to UTC time, or as a date and time in other (user defined) time zones. Negative time offsets refer to timezones West of Greenwich, and positive offsets refer to timezones East of Greenwich: these offsets represent the zones’ local time differences with UTC time. Timezone offsets are always computed modulo 12 hours, as timezones may at most differ 12 hours from UTC.

DateTime objects always contain date and time points in seconds since the beginning of the ’epoch’ (midnight Jan 1, 1970 UTC). The C function time(2) returns this value for the current date and time (assuming that the computer’s clock has properly been synchronized).

Daylight Saving Time (DST), when defined for DateTime objects’ zones is automatically applied when DST is active. E.g., when a DateTime object’s time represents 12:00 hr., and it zone’s DST (using a standard +1 hour shift) becomes active tomorrow, then the object’s time shows 13:00 hr. when 1 day is added to its current date/time.

Handling time is complex. The C function time(2) returns the time in seconds since the beginning of the epoch. Beyond that, simplicity ends, and the reader is referred to Eric S. Raymond’s (2007) coverage of the complexities of handling time: see http://www.catb.org/esr/time-programming (this document is also included in bobcat’s repository).

TIMEZONES

DateTime’s nested class Zone is used to specify time zones. Time zones have various configurable components:

DataTime::Zone’s class offers a variety of means for specifying time zones with or without support for DST. Time shifts can be positive or negative and are specified in hours and optionally minutes. E.g., -8 or -8:00 for a timezone 8 hours earlier than UTC (noon at UTC becomes 04:00 in this timezone) or +5:30 for a time zone later than UTC (noon at UTC becomes 17:30 in that timezone). Timezones can be given names (which may or may not be the names of the ’standard’ time zones like CET or PST) but can also be constructed by providing

NAMESPACE

FBB
All constructors, members, operators and manipulators, mentioned in this man-page, are defined in the namespace FBB.

INHERITS FROM

-

ENUMS

The class DateTime defines the following enums:

DateTime::Month
This enumeration has the following values which are ordered using the default C++ enum values:

Standard 3-letter abbreviations can also be used (e.g., DateTime::Jul):

DateTime::Relative
This enumeration is used with the setMonth() member (see below). It has the following values:

DateTime::TimeFields
This enumeration has the following values which can be bit_or-ed when calling the member setFields():

DateTime::TimeType
This enumeration has the following values:

DateTime::Weekday
This enumeration has the following values which are ordered using the default C++ enum values:

Standard 3-letter abbreviations can also be used (e.g., DateTime::Wed):

TEXTUAL TIME REPRESENTATIONS

DateTime objects may also be defined using textual time-representations. In addition, the date/time represented by DateTime objects may be altered using textual time representations extracted from istreams.

The following time formats (shown by example) are recognized:

ZONE CONSTRUCTORS

The class DateTime::Zone defines how time zones can be defined and used. Time zone objects are expected by several DateTime constructors and by the DateTime::setZone member. The class DateTime::Zone supports the following constructors:

The argument name refers to a predefined zone name. By default the zone CET is available. Other zone names can be defined using the members DateTime::Zone::read and DateTime::Zone::store;

The argument specification refers to a zone specification that’s available under the directory referred to by /etc/localtime (e.g., /usr/share/zoneinfo). The specification must start with a colon, followed by the name of an entry that is found below /usr/share/zoneinfo. E.g, to specify the Europe/Amsterdam zone use DateTime::Zone{ ":Europe/Amsterdam" }. If the specified zone uses DST then a +1 hour DST shift is used;

The argument shift defines the zone’s shift in hours and minutes relative to UTC. Shift’s format is [+-]hh[:mm], where negative time shifts refer to time zones West of Greenwich and positive time shifts to time zones East of Greenwich. No DST is used;

The argument shift defines the zone’s shift in hours and minutes relative to UTC. Shift’s format is [+-]hh[:mm], where negative time shifts refer to time zones West of Greenwich and positive time shifts to time zones East of Greenwich. The dstSpec argument defines the time shift to use when DST is active. It uses the same format as shift, or = may be specified, in which case the (standard) DST shift of +1:00 hour is used;

Like the previous constructor, but in addition beginDST and endDST are used to define the date/time interval in which the DST is active. These arguments use the following format:

Mon, weekSpec Day [hh[:mm]]

For Mon a standard 3-letter month specification is used, like "Jan" for January, for Day a standard 3-letter day specification is used, like "Sun" for Sunday (cf. the standard 3-letter abbreviations listed in the section ENUMS).

The weekSpec defines the number of the week of the month containing Day. Use 1st, 2nd, 3rd, 4th, 5th or last. Here, 1st refers to the first week having day Day, while last and 5th refer to the last week (so not necessarily the 5th week) having day Day.

The time specification defines the time when the DST starts or ends. Some examples:

Mar, last Sunday 02:00
Oct, 1st Monday 03:00

Copy and move constructors as well as assignment operators are available.

ZONE MEMBERS

The constructors’ zone specifications are converted to a form recognized by the function tzset(3). This member returns the converted string;

If DST is active for a local date/time point then the DST shift in seconds is returned by this member. Use the member DateTime::dst() to determine whether or not DST is active.

The zone’s time shift relative to UTC in seconds. Negative values represent zone shifts West of Greenwich, positive values represent zone shifts East of Greenwich.

ZONE STATIC MEMBERS

The time zone object named name is returned. E.g., to retrieve the time zone object for CET use DateTime::Zone::get("CET"). If the named zone has not been defined an std::exception is thrown;

A time zone object is defined and stored as zone name. Zone names must consists of (at least three) letters. The ellipsis should be replaced by the arguments of the Zone constructors (except for the arguments of the first, copy, and move constructors). A reference to the newly defined Zone object is returned;

The TZ specification that is active when the program using DateTime objects starts. It returns the return value of getenv("TZ") or an empty string if the TZ environment variable was not defined;

Time zone specifications as expected by the above store member are read from the file named fname (one specification per line, using blanks between the elements, and inserting until between the dstBegin and dstEnd specifications (if provided)). For cosmetic reasons a colon may be appended to zone names. Examples:

ccu: +5:50
nyc: -5:00 +1:00
cal: -8:00 =
eet: +2:00 +1:00 Mar, last Sun 02:00 until Oct, last Sun 02:00
same: +2:00 = Mar, last Sun 02:00 until Oct, last Sun 02:00

If the first character on a line is a hash-tag (#) or if a line only contains blanks it is ignored;

Returns this computer’s zone shift in seconds.

DATETIME CONSTRUCTORS

The object’s date/time is initialized to the current date and time. By default UTC time (no zone-shift or dst correction) is used. When specifying LOCALTIME, the computer’s timezone and DST correction are used;

The object’s local time is initialized to the current UTC time to which a zone shift of zoneMinutes minutes is added. DST is not used. Note that by specifying a zone shift in, e.g., std::chrono::hours a conversion to minutes is automatically applied;

The object’s local time is initialized to the the current UTC time to which a zone shift defined by the Zone argument is added;

The object uses time to set its UTC time in seconds since the epoch. When LOCALTIME is specified its local time is obtained by applying the computer’s timezone and DST correction;

The object’s uses time to set its UTC time in seconds since the epoch. Its local time is then obtained by adding a zone shift of zoneMinutes minutes. DST is not used.;

The object’s uses time to set its UTC time in seconds since the epoch. Its local time is then obtained by adding a zone shift as defined by the Zone argument.

The tm argument is a struct defined as:

struct tm
{
int tm_sec; // seconds 0..59, or 60: leap second
int tm_min; // minutes 0..59
int tm_hour; // hours 0..23
int tm_mday; // day of the month 1..31
int tm_mon; // month 0..11
int tm_year; // year see below!!
int tm_wday; // day of the week 0..6
int tm_yday; // day in the year 0..365
int tm_isdst; // daylight saving time
// > 0: yes, 0: no, < 0: unknown
};

Values outside of these ranges may be specified to compute points in time in the future or in the past. E.g., tm_hour == 30 is normalized to tm_hour == 6 of the next day.

Note that functions like mktime(3) expect tm_year to be specified relative to 1900. The DateTime constructor does not expect this: the tm_year field should be the specified as the (calendar) year, e.g., 2019.

Using tm’s fields the object’s date/time is initialized to either UTC or LOCALTIME. The DST, day of the year, and day of the week fields of the tm argument are ignored;

The fields of ts (see the previous constructor) define a LOCALTIME time point in the specified Zone.

The object’s date/time is initialized to either UTC or LOCALTIME using DateTime’s supported textual time representations. The time representation without explicit timezone shift (e.g., Mon Dec 3 13:29:11 2018 is interpreted as UTC time when type == UTC is specified, otherwise as local time (type == LOCALTIME), using the computer’s time zone and DST correction). The other textual time representations provide zone specifications which are used to obtain the requested time representation (e.g., when UTC is requested, and the textual time specification contains 13:29:11 +0100 then the object’s (UTC) time is set to 12:29:11). DST specifications are ignored, except when named time zones are used, as in on Dec 3 13:29:11 CET 2018;

The object’s date/time is initialized to either UTC or LOCALTIME extracting a supported textual time representation from in as destribed at the DateTime(std::string const &timeStr, ...) constructor. The std::istream &in stream may also be a rvalue reference.

Same as the previous constructor, but receeiving an r-value reference as its first argumnt.

Copy and move constructors are available.

OVERLOADED OPERATORS

All class-less overloaded operators are defined in the FBB namespace. All overloaded operators modifying DateTime objects support ’commit or roll-back’: if the operation cannot be performed an exception is thrown, without modifying the destination object.

Inserts dt’s date/time into str using the zone-less supported textual time representation (e.g., Mon Dec 3 13:29:11 2018);

Extracts a supported textual time representation from str and assigns the implied date/time point to dt;

Returns true if left represents the same UTC time as right;

Returns true if left represents a different UTC time as right;

Returns true if left represents an earlier UTC time as right;

Returns true if left represents an earlier or equal UTC time as right;

Returns true if left represents a later UTC time as right;

Returns true if left represents a later or equal UTC time as right.

Returns a copy of obj to which seconds were added;

Returns a copy of obj to which the tm_sec, tm_min, tm_hour, tm_mday, tm_mon and tm_year fields of ts were added;

Caveat: note that this is an addition operation: if you want to add 1 year specify ts.tm_year = 1, and not the finally expected calendar year!

Adds seconds to the time represented by the current object;

Adds the tm_sec, tm_min, tm_hour, tm_mday, tm_mon and tm_year fields of ts to the time represented by the current object.

Caveat: note that this is an addition operation: if you want to add 1 year specify ts.tm_year = 1, and not the finally expected calendar year!

Returns a copy of obj to which seconds were subtracted;

Returns a copy of obj to which the tm_sec, tm_min, tm_hour, tm_mday, tm_mon and tm_year fields of ts were subtracted.

Caveat: note that this is an addition operation: if you want to subtract 1 year specify ts.tm_year = 1, and not the finally expected calendar year!

Subtracts seconds from the time represented by the current object;

Subtracts the tm_sec, tm_min, tm_hour, tm_mday, tm_mon and tm_year fields of ts from the time represented by the current object.

Caveat: note that this is an addition operation: if you want to subtract 1 year specify ts.tm_year = 1, and not the finally expected calendar year!

E.g., the following program fragment displays midnight, December 31, 1969:
DateTime dt{ 0, DateTime::UTC }; // set UTC at begin of era

tm era{ 0 }; // define a tm with tm_mday == 1
era.tm_mday = 1;

dt -= era; // subtract from dt and display
cout << dt << endl;

Copy and move assignment operators are available.

MEMBER FUNCTIONS

All members returning a time-element do so according to the latest time-representation (i.e., UTC, LOCALTIME, or using an explicitly set display zone shift value). All members returning numeric values use 0 as their smallest return value, except for the ...Nr() members, which start at 1.

Returns true if the current object’s time includes a DST shift, otherwise false is returned.

Returns the number of hours represented by the current object’s time (0-23);

Returns the number of minutes represented by the current object (0-59);

Returns the Month represented by the current object;

Returns the day number of the month represented by the current object’s date (1-31);

Returns the current object’s date/time displayed according to the RFC 2822 format. This format is used, e.g., by the date -R command (cf. date(1)). For example:

Mon, 3 Dec 2018 13:49:10 +0100

Returns the current object’s date/time displayed according to the the RFC 3339 format. This format is used, e.g., by the date --rfc-3339=seconds command (cf. date(1)). For example:

2018-12-03 13:29:11+01:00

Returns the number of seconds represented by the current object’s time (0-59, but 60 and 61 may occur at leap seconds). Note that this member does not return the current UTC time in seconds (since the epoch): if that’s required use the member utcSeconds;

Assigns dayNr to the day number of the object’s current month.

Caveat: since day numbers start at 1, passing 0 or negative values to setDay results in resetting the objects date to an earlier month;

Assigns the fields of ts indicated by a bit_or combination of TimeFields fields’ value to the corresponding current object’s date/time tm fields;

Assigns hours to the number of hours of the current object’s time;

Assigns minutes to the number of minutes of the current object’s time;

Assigns month to the current object’s month. January is represented by 0, December by 11. Smaller or larger values refer to previous or future years;

Assigns month to the current object’s month;

Assigns month to the current object’s month. By default the month of the current year is updated (where == THIS_YEAR). Use LAST to ensure that the month is set before the current object’s month (e.g., if the current month is JUNE, then requesting AUGUST, LAST will decrement the object’s year, but requesting MAY, LAST won’t). Analogously, when specifying DateTime::NEXT the resulting month is set after the current object’s month.

Caveat: If the day number of the current month exceeds the number of days in the requested month, the object’s month and day number are updated to the next month. E.g., if the current day number equals 31, and NOVEMBER is requested, then the object’s date is updated to December 1;

Assigns seconds to the number of seconds of the current object’s time;

Assigns utcSeconds as the number of seconds since the epoch to the current object’s UTC time;

Assigns day to the current object’s day of the week. By default (where == NEXT) the day will be the next occurrence of day (maybe the current week, maybe in the next week: if the current day equals Monday and Friday is specified, the current week is used. If the current day equals Friday and Monday is requested the next week is used). By specifying where = LAST the day will be most recent occurrence of day: if the current day equals Friday and Monday is requested, the current week is used. By specifying where = THIS_WEEK then day is selected in the current week;

Assigns year to the current object’s date. Note that year is the actual calendar year, and not, e.g., the year relative to the beginning of the epoch or 1900;

The current object’s time zone is set to zone. This does not alter the object’s UTC time, but merely its time zone. If the object represented UTC time before calling setZone it will represent a local time after calling setZone;

Swaps the current and other DateTime object;

Returns a DateTime object using the current object’s UTC time, applying the computer’s default time zone;

Returns a pointer to the object’s current tm representing its object’s broken down time elements. If the object holds a local time the tm struct represents the local time (if applicable: including a DST shift), otherwise it represents UTC;

Returns a copy of the current object representing its UTC time;

Returns the current object’s (UTC) time in seconds since the epoch;

Returns the current object’s Weekday value;

Returns the week number of the current object’s date. Week numbers are numbers of complete weeks. If Jan 1st is a Sunday then the week numbers of Jan 1st through Jan 6th are returned as 1, otherwise the week numbers of Jan 1st through the date of the first Saturday of the year (which could very well be Jan 1st) are returned as 52;

Returns the year of the current object’s date (note: this is the actual calendar year, not the year since the epoch or relative to 1900);

Returns the day in the year of the current object’s date. January 1st is returned as 0;

Returns the day in the year of the current object’s date. January 1 is returned as 1;

Returns the current object’s Zone object.

STATIC MEMBER

This static member is primarily used for debugging purposes. It displays the values of ts’s fields, preceded by label and ": ".

EXAMPLES

Many examples illustrating the use of DateTime objects are provided in the source archive’s bobcat/datetime/driver directory. Here is an example:
#include <iostream>

#include "../datetime"

using namespace std;
using namespace FBB;

void show(DateTime const &dt, char const *label)
{
cout <<
label << ": " << dt << "\n"
"dst: " << dt.dst() << "\n"
"hh:mm:ss: " << dt.hours() << ’:’ << dt.minutes() << ’:’ <<
dt.seconds() << "\n"
"year-month-monthdaynr: " << dt.year() << ’-’ << dt.month() << ’-’ <<
dt.monthDayNr() << "\n"
"weekday/weeknr/yearday/yeardaynr: " <<
dt.weekday() << ’/’ << dt.weekNr() << ’/’ <<
dt.yearDay() << ’/’ << dt.yearDayNr() << "\n"
"\n";
}

int main()
{
time_t now = time(0);

DateTime utc{ now, DateTime::UTC };
show(utc, "Current UTC time");

DateTime local{ utc.thisTime() };

cout << "The COMPUTER’S LOCAL TIME: " << local << ’\n’;

DateTime dt{ now, DateTime::LOCALTIME };
show(dt, "Current LOCAL TIME");

DateTime utc2{ dt.utc() };
cout << "UTC obtained from LOCAL TIME: " << utc2 << ’\n’;

DateTime jan1{ "2019-01-01 13:00:00+01:00", DateTime::LOCALTIME };
cout << "Jan 1, 1919, 13:00h: " << jan1 << ’\n’;
show(jan1, "Jan 1, details:");

cout << "\nOptionally rerun specifying another time zone specification\n"
"\n";
}

FILES

bobcat/datetime defines the class interface.

SEE ALSO

bobcat(7), Exception(3bobcat), gmtime_r(3), localtime_r(3), mktime(3), time(2), tzset(3),
http://www.catb.org/esr/time-programming,
https://www.timeanddate.com/time/dst/events.html

BUGS

See bobcat’s changelog file for an overview of members that were discontinued at release 5.00.00.
The class DateTime assumes that time(2) returns the time in UTC.
English is used/expected when specifying named date components.

BOBCAT PROJECT FILES

BOBCAT

Bobcat is an acronym of ’Brokken’s Own Base Classes And Templates’.

COPYRIGHT

This is free software, distributed under the terms of the GNU General Public License (GPL).

AUTHOR

Frank B. Brokken (f.b.brokken [AT] rug.nl).