# man calendar () - Local and universal time, day-of-the-week, date and time conversions

## NAME

calendar - Local and universal time, day-of-the-week, date and time conversions

## DESCRIPTION

This module provides computation of local and universal time, day-of-the-week, and several time conversion functions.

Time is local when it is adjusted in accordance with the current time zone and daylight saving. Time is universal when it reflects the time at longitude zero, without any adjustment for daylight saving. Universal Coordinated Time (UTC) time is also called Greenwich Mean Time (GMT).

The time functions local_time/0 and universal_time/0 provided in this module both return date and time. The reason for this is that separate functions for date and time may result in a date/time combination which is displaced by 24 hours. This happens if one of the functions is called before midnight, and the other after midnight. This problem also applies to the Erlang BIFs date/0 and time/0, and their use is strongly discouraged if a reliable date/time stamp is required.

All dates conform to the Gregorian calendar. This calendar was introduced by Pope Gregory XIII in 1582 and was used in all Catholic countries from this year. Protestant parts of Germany and the Netherlands adopted it in 1698, England followed in 1752, and Russia in 1918 (the October revolution of 1917 took place in November according to the Gregorian calendar).

The Gregorian calendar in this module is extended back to year 0. For a given date, the gregorian days is the number of days up to and including the date specified. Similarly, the gregorian seconds for a given date and time, is the the number of seconds up to and including the specified date and time.

For computing differences between epochs in time, use the functions counting gregorian days or seconds. If epochs are given as local time, they must be converted to universal time, in order to get the correct value of the elapsed time between epochs. Use of the function time_difference/2 is discouraged.

## DATA TYPES

```date() = {Year, Month, Day}
Year = int()
Month = 1..12
Day = 1..31
Year cannot be abbreviated. Example: 93 denotes year 93, not 1993.
Valid range depends on the underlying OS.
The date tuple must denote a valid date.
```

time() = {Hour, Minute, Second} Hour = 0..23 Minute = Second = 0..59

## EXPORTS

date_to_gregorian_days(Date) -> Days

date_to_gregorian_days(Year, Month, Day) -> Days

Types
Date = date()

Days = int()

This function computes the number of gregorian days starting with year 0 and ending at the given date.

datetime_to_gregorian_seconds({Date, Time}) -> Seconds

Types
Date = date()

Time = time()

Seconds = int()

This function computes the number of gregorian seconds starting with year 0 and ending at the given date and time.

day_of_the_week(Date) -> DayNumber

day_of_the_week(Year, Month, Day) -> DayNumber

Types
Date = date()

DayNumber = 1..7

This function computes the day of the week given Year, Month and Day. The return value denotes the day of the week as 1: Monday, 2: Tuesday, and so on.

gregorian_days_to_date(Days) -> Date

Types
Days = int()

Date = date()

This function computes the date given the number of gregorian days.

gregorian_seconds_to_datetime(Seconds) -> {Date, Time}

Types
Seconds = int()

Date = date()

Time = time()

This function computes the date and time from the given number of gregorian seconds.

is_leap_year(Year) -> bool()

This function checks if a year is a leap year.

last_day_of_the_month(Year, Month) -> int()

This function computes the number of days in a month.

local_time() -> {Date, Time}

Types
Date = date()

Time = time()

This function returns the local time reported by the underlying operating system.

local_time_to_universal_time({Date1, Time1}) -> {Date2, Time2}

This function converts from local time to Universal Coordinated Time (UTC). Date1 must refer to a local date after Jan 1, 1970.

### Warning:

This function is deprecated. Use local_time_to_universal_time_dst/1 instead, as it gives a more correct and complete result. Especially for the period that does not exist since it gets skipped during the switch to daylight saving time, this function still returns a result.

local_time_to_universal_time_dst({Date1, Time1}) -> [{Date, Time}]

Types
Date1 = Date = date()

Time1 = Time = time()

This function converts from local time to Universal Coordinated Time (UTC). Date1 must refer to a local date after Jan 1, 1970.

The return value is a list of 0, 1 or 2 possible UTC times:

[]: For a local {Date1, Time1} during the period that is skipped when switching to daylight saving time, there is no corresponding UTC since the local time is illegal - it has never happened.
[DstDateTimeUTC, DateTimeUTC]: For a local {Date1, Time1} during the period that is repeated when switching from daylight saving time, there are two corresponding UTCs. One for the first instance of the period when daylight saving time is still active, and one for the second instance.
[DateTimeUTC]: For all other local times there is only one corresponding UTC.

now_to_local_time(N) -> {Date, Time}

Types
Now -- see erlang:now/0

Date = date()

Time = time()

This function returns local date and time converted from the return value from erlang:now().

now_to_universal_time(N) -> {Date, Time}

now_to_datetime(N) -> {Date, Time}

Types
Now -- see erlang:now/0

Date = date()

Time = time()

This function returns Universal Coordinated Time (UTC) converted from the return value from erlang:now().

seconds_to_daystime(Seconds) -> {Days, Time}

Types
Seconds = Days = int()

Time = time()

This function transforms a given number of seconds into days, hours, minutes, and seconds. The Time part is always non-negative, but Days is negative if the argument Seconds is.

seconds_to_time(Seconds) -> Time

Types
Seconds = int() < 86400

Time = time()

This function computes the time from the given number of seconds. Seconds must be less than the number of seconds per day (86400).

time_difference(T1, T2) -> {Days, Time}

This function returns the difference between two {Date, Time} tuples. T2 should refer to an epoch later than T1.

### Warning:

This function is obsolete. Use the conversion functions for gregorian days and seconds instead.

time_to_seconds(Time) -> Seconds

Types
Time = time()

Seconds = int()

This function computes the number of seconds since midnight up to the specified time.

universal_time() -> {Date, Time}

Types
Date = date()

Time = time()

This function returns the Universal Coordinated Time (UTC) reported by the underlying operating system. Local time is returned if universal time is not available.

universal_time_to_local_time({Date1, Time1}) -> {Date2, Time2}

Types
Date1 = Date2 = date()

Time1 = Time2 = time()

This function converts from Universal Coordinated Time (UTC) to local time. Date1 must refer to a date after Jan 1, 1970.

valid_date(Date) -> bool()

valid_date(Year, Month, Day) -> bool()

Types
Date = date()

This function checks if a date is a valid.

## Leap Years

The notion that every fourth year is a leap year is not completely true. By the Gregorian rule, a year Y is a leap year if either of the following rules is valid:

*
Y is divisible by 4, but not by 100; or
*
Y is divisible by 400.

Accordingly, 1996 is a leap year, 1900 is not, but 2000 is.

## Date and Time Source

Local time is obtained from the Erlang BIF localtime/0. Universal time is computed from the BIF universaltime/0.

The following facts apply:

*
there are 86400 seconds in a day
*
there are 365 days in an ordinary year
*
there are 366 days in a leap year
*
there are 1461 days in a 4 year period
*
there are 36524 days in a 100 year period
*
there are 146097 days in a 400 year period
*
there are 719528 days between Jan 1, 0 and Jan 1, 1970.

## AUTHOR

```Peter Högfeldt - support@erlang.ericsson.se
```