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# Difference between revisions of "Convert Epoch Seconds to DMYHMS"

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## 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.

## 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
```

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],_} =
{ok,[2004,4,26,18,26,18,500],[]}
```

## 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)).
```

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],_} =