Tonight we count down from 10 to 0 at midnight and bring in the New Year.
But will it be 2019 when we reach 0? What if there is a leap second?
Quick -- let's check the list of leap seconds provided by the IETF.
No -- it looks good, January 1, 2017 was the last one, and that list is good until June 28, 2019.
Two years ago, things were different.
At "midnight" (UTC), it became 2017.
~ $ raku -e 'say DateTime.new("2017-01-01T00:00:00Z").year'
2017
But actually, TWO seconds had passed since 11:59:59 the night (and year) before!
~ $ raku -e 'say DateTime.new("2017-01-01T00:00:00Z") - DateTime.new("2016-12-31T23:59:59Z")'
2
At the instant that it became 2017, there was a leap second. The value returned by now would have indicated that.
Tonight will be fine though:
~ $ raku -e 'say DateTime.new("2019-01-01T00:00:00Z") - DateTime.new("2018-12-31T23:59:59Z")'
1
If we want to be more explicit, we can turn
a DateTime into an Instant and back again by using posix
and from-posix -- the latter takes a boolean value as a
second parameter to indicate whether or not leap seconds
should be taken into account.
my $date = DateTime.new("2017-01-01T00:00:00Z");
my $instant = Instant.from-posix: $date.posix, True;
my $instant2 = Instant.from-posix: $date.posix;
say $instant.DateTime.year;
say $instant2.DateTime.year;
2016 2017
I wonder how many languages have native support for leap seconds?
It looks like not many. Which is too bad. If leap seconds catch you off guard, weird things can happen.