Re: Back and forth UTC dates

[Dirk Laurie <dirk.laurie@...>]

I'm top-posting since I don't wish to touch that.

It looks like Perl.

2017-03-28 13:36 GMT+02:00 Francisco Olarte <folarte@peoplecall.com>:
> Javier:
>
> On Tue, Mar 28, 2017 at 12:32 PM, Javier Mr <javiersdevmail@ymail.com> wrote:
>> I'm having a little bit of trouble converting dates in UTC to timestamps and back to dates.
>
> This is because os.time() needs table in local time. You do not have a
> function to work with the date in UTC.
>
>> I obtain the ITC timestamp as os.time( os.date( "!*t" ) ); back when converting it back to date doesn't matter is I use "!*t" or "*t"; I always get the hour wrong, either one hour less or one hour more.
>
> First, if you do not pass a second argument to date, you are doing it
> wrong ( just call os.time() ). Assuming that was a typo ( and you
> meant os.time( os.date( "!*t", anyts ) ) what you are doing there is
> telling "format anyts as a timestamp and then parse it back as local".
> This is what you do to TRY to find the current offset with UTC in your
> local zone.
>
> Anyway, timestamps ( the numbers ) are not UTC or local, you do not
> need to correct them. They are ( in POSIX systems ) defined as the
> number of seconds since the epoch. The time value is the same for all
> timezones.
>
> Converting timestamps to dates is easy. To convert dates to timestamps
> is where you have a problem. What you can try to do is:
>
> 1.- Have a date ( table ) in utc, date_utc - convert it to a local ts,
> ts_local - It is erroneous.
> 2.- Use the ts_local to find the time zone offset - os.date("%z", ts_local)
> 3.- Parse the timezone and correct ts_local
>
> Or you can write a very small function to build the timestamp using
> tz_date. The usual way is to use a couple of tables. I routenly use
> this in perl:
>
> my @days_per_month = (0,31,28,31,30,31,30,31,31,30,31,30,31); # NB,
> dec=#12 not used
> ---- Acum days counts the days BEFORE day 1 of the month.
> my @acum_days = (0,0);
>
> for(1..12) {
>     $acum_days[$_+1]=$acum_days[$_]+$days_per_month[$_];
> }
>
> sub ymd2days($$$) {
>     my ($y,$m,$d) = @_;
>     my $days = $d + $acum_days[$m] + $y * 365 + int($y/4)
> -int($y/100)+ int($y/400);
>     my $bisiesto = not (  ($y%4) or (not $y%100 and $y%400) );
>     --$days if ($bisiesto and $m<3);
>     return $days;
> }
>
> ( leap year in spanish is "año bisiesto" )
>
> use constant EPOCH_DAYS =>  1970 * 365 +
> int(1970/4)-int(1970/100)+int(1970/400) +1;
>
> >From this you can turn a date to a timestamp : (
> seconds+60*(minutes+60*(hours+24*(ymd2days(y,m,d)-EPOCH_DAYS))))
>
> Porting it to lua is not that difficult and left as an exercise to the
> reader ( I can do it if you have troubles, just not now ).
>
> Francisco Olarte.
>