Convert timestamp to datetime in erlang - date

Question

How can I convert a timestamp (number of milliseconds since 1 Jan 1970..., aka epoch) to Date or DateTime format in Erlang? Something like {Year,Month,Day}.

Answer 1

Roughly:
msToDate(Milliseconds) ->
BaseDate = calendar:datetime_to_gregorian_seconds({{1970,1,1},{0,0,0}}),
Seconds = BaseDate + (Milliseconds div 1000),
{ Date,_Time} = calendar:gregorian_seconds_to_datetime(Seconds),
Date.

Answer 2

It just so happens that I have a github gist with a bunch of datetime utilities for exactly this purpose: http://gist.github.com/104903. Calendar has most of the low level plumbing for this stuff.
-module(date_util).
-compile(export_all).
epoch() ->
now_to_seconds(now())
.
epoch_hires() ->
now_to_seconds_hires(now())
.
now_to_seconds({Mega, Sec, _}) ->
(Mega * 1000000) + Sec
.
now_to_milliseconds({Mega, Sec, Micro}) ->
now_to_seconds({Mega, Sec, Micro}) * 1000
.
now_to_seconds_hires({Mega, Sec, Micro}) ->
now_to_seconds({Mega, Sec, Micro}) + (Micro / 1000000)
.
now_to_milliseconds_hires({Mega, Sec, Micro}) ->
now_to_seconds_hires({Mega, Sec, Micro}) * 1000
.
epoch_gregorian_seconds() ->
calendar:datetime_to_gregorian_seconds({{1970,1,1}, {0,0,0}})
.
now_to_gregorian_seconds() ->
epoch_to_gregorian_seconds(now())
.
epoch_to_gregorian_seconds({Mega, Sec, Micro}) ->
epoch_to_gregorian_seconds(now_to_seconds({Mega, Sec, Micro}));
epoch_to_gregorian_seconds(Now) ->
EpochSecs = epoch_gregorian_seconds()
, Now + EpochSecs
.
gregorian_seconds_to_epoch(Secs) ->
EpochSecs = epoch_gregorian_seconds()
, Secs - EpochSecs
.
date_to_epoch(Date) ->
datetime_to_epoch({Date, {0,0,0} })
.
datetime_to_epoch({Date, Time}) ->
gregorian_seconds_to_epoch(
calendar:datetime_to_gregorian_seconds({Date, Time}))
.
is_older_by(T1, T2, {days, N}) ->
N1 = day_difference(T1, T2)
, case N1 of
N2 when (-N < N2) ->
true;
_ ->
false
end
.
is_sooner_by(T1, T2, {days, N}) ->
case day_difference(T1, T2) of
N1 when N > N1 ->
true;
_ ->
false
end
.
is_time_older_than({Date, Time}, Mark) ->
is_time_older_than(calendar:datetime_to_gregorian_seconds({Date, Time})
, Mark);
is_time_older_than(Time, {DateMark, TimeMark}) ->
is_time_older_than(Time
, calendar:datetime_to_gregorian_seconds({DateMark, TimeMark}));
is_time_older_than(Time, Mark) when is_integer(Time), is_integer(Mark) ->
Time < Mark
.
day_difference({D1, _}, D2) ->
day_difference(D1, D2);
day_difference(D1, {D2, _}) ->
day_difference(D1, D2);
day_difference(D1, D2) ->
Days1 = calendar:date_to_gregorian_days(D1)
, Days2 = calendar:date_to_gregorian_days(D2)
, Days1 - Days2
.
is_time_sooner_than({Date, Time}, Mark) ->
is_time_sooner_than(calendar:datetime_to_gregorian_seconds({Date, Time})
, Mark);
is_time_sooner_than(Time, {DateMark, TimeMark}) ->
is_time_sooner_than(Time
, calendar:datetime_to_gregorian_seconds({DateMark, TimeMark}));
is_time_sooner_than(Time, Mark) when is_integer(Time), is_integer(Mark) ->
Time > Mark
.
subtract(Date, {days, N}) ->
New = calendar:date_to_gregorian_days(Date) - N
, calendar:gregorian_days_to_date(New)
.
add(Date, {days, N}) ->
New = calendar:date_to_gregorian_days(Date) + N
, calendar:gregorian_days_to_date(New)
.

Answer 3

OTP 21.0 added this function
calendar:system_time_to_universal_time(Time, TimeUnit) -> datetime()
Types
Time = integer()
TimeUnit = erlang:time_unit()
Converts a specified system time into universal date and time.
Example:
> os:system_time(1000).
1598512151718
> calendar:system_time_to_universal_time(1598512151718, 1000).
{{2020,8,27},{7,9,11}}
Refrence: https://erlang.org/doc/man/calendar.html#system_time_to_universal_time-2