Re: What is the best way to get a list of all Mondays until the end of the year?

[Rhandom]

in reply to What is the best way to get a list of all Mondays until the end of the year?

Golfing - with no modules:

perl -e '@N=gmtime;$t=time+86400*(1-$N[6]);while(1){@n=gmtime($t+=7*86
+400);last if $n[5]!=$N[5]; printf("%d-%02d-%02d\n",$n[5]+1900,$n[4]+1
+,$n[3])}'

Human readable and explained:

perl -e '
my @N = gmtime;     # today (calculations in gmtime to avoid tz offset
+s)

my $t = time + 86400 * (1 - $N[6]);    # calculate the time of Monday 
+(1)

while(1) { 
      my @n = gmtime($t += 7*86400); # advance a week at a time

      last if $n[5] != $N[5];           # done if the year doesn't mat
+ch today's

     printf ("%d-%02d-%02d\n", $n[5]+1900, $n[4]+1, $n[3]); # print re
+sult
}'

Really you should look at DateTime or Date::Manip or Date::Calc or ... - but sometimes it makes sense not to.

[Krambambuli]

Just a side node; this would work most of the time, but not always. A week is _not_ always 7*86400 seconds, a day is _not_ always 86400 seconds - when daylight saving comes to play, this offset may give you a false Monday (if you're making the calculation at a unwise chosen time ;) )

Update: daylight saving changes.

[broomduster]

What you say is true for localtime, but not for gmtime (which is what the Rhandom's example uses). As the doc says, there is no DST for GMT.

[Krambambuli]

Sure - but the Mondays we're interested in would probably need to be in local timezone and DST, not GMT. I'm not really sure the code would fail, but I wouldn't bet I get the correct list of dates regardless when the calculation is done either.

I'm not sure the back-and-forth between localtime and gmtime works OK for this calculation - but consider that the Mondays surrounding a DST change date are 7*86400 +- 3600 seconds apart in 'real life' - using 7*86400 instead might place me a day ahead or behind if I'm somehwere near midnight when making the calculation.

Just a warning about the not-at-all-trivial datetime calculations.