Convert Epoch Seconds to DMYHMS
From Erlang Community
[edit] Problem
You have a date and time in Erlang Epoch seconds (i.e., Gregorian calendar year 0 seconds), and you want to calculate the individual DMYHMS values from it.
[edit] Solution
In recipe TimeToday we did exactly this but only for the current date. How about if we wanted to get the date, and time (DMYHMS) associated to a particular number of seconds? Once more we would use gre.
1> Seconds = 1083022458.
1083022458
2> DateTime = calendar:gregorian_seconds_to_datetime(Seconds).
{{34,4,26},{23,34,18}}
3> {{Year, Month, Day}, {Hour, Min, Sec}} = DateTime.
{{34,4,26},{23,34,18}}
4> io:fwrite("Today's Date is ~2B/~2B/~4..0B ~2B:~2.10.0B:~2.10.0B\n",
[Month, Day, Year, Hour, Min, Sec]).
Today's Date is 4/26/0034 23:34:18
ok
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In recipe TimeToday we discussed how to convert a date structure to a string. Convert in the opposite direction is also possible, and useful. Unfortunately, there is not a lot of built-in Erlang plumbing to do so. However, if we are very sure of the format we can easily extract the data we need using Erlang's ever-helpful io and io_lib modules:
5> Some_Date_String = "2004-04-26T18:26:18-0500".
"2004-04-26T18:26:18-0500".
6> {ok, [YYY,MMM,DD,HH,MM,SS,ZZ],_} =
6> io_lib:fread("~4d-~2d-~2dT~2d:~2d:~2d-~4d", Some_Date_String).
{ok,[2004,4,26,18,26,18,500],[]}
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[edit] Symmetric conversion
The above conversion writes directly to the console, but sometimes, one would like an approach in which functions return the converted string and given a string, seconds are returned:
epoch_to_mdyhms(Seconds) -> DateString mdyhms_to_epoch(DateString) -> Seconds |
When creating such a solution, symmetry is an important property to strive after. Intuitively, one expects the following property to hold for conversions
prop_epoch_mdyhms_identity() ->
?FORALL(Seconds,seconds(),
mdyhms_to_epoch(epoch_to_mdyhms(Seconds)) == Seconds).
seconds() ->
?LET(I,largeint(),abs(I)).
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However, that property clearly is violated in the code above, which we can observe by manually looking at the output produced.
A symmetric and testable solution is given by:
epoch_to_mdyhms(Seconds) ->
{{Year, Month, Day}, {Hour, Min, Sec}} =
calendar:gregorian_seconds_to_datetime(Seconds),
lists:flatten(
io_lib:fwrite("~2B/~2B/~4..0B ~2B:~2.10.0B:~2.10.0B",
[Month, Day, Year, Hour, Min, Sec])).
mdyhms_to_epoch(DateString) ->
{ok,[Month, Day, Year, Hour, Min, Sec],_} =
io_lib:fread("~d/~d/~d ~d:~d:~d", DateString),
calendar:datetime_to_gregorian_seconds({{Year,Month,Day},{Hour,Min,Sec}}).
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