Fix date calculation (again) (#2624)

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## About The Pull Request
My math was all over the place. This makes it so that there's only one
day in year day and months don't start on the second. In my defense it's
kinda hard to test this.

## Why It's Good For The Game
Time consistency

## Changelog

:cl:
fix: IC/sector months now actually start on the first and there's only
one year day
/:cl:

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code/__HELPERS/time.dm

@@ -22,7 +22,7 @@
 	//International Fixed Calendar format (https://en.wikipedia.org/wiki/International_Fixed_Calendar)
 	var/days_since = round(realtime / (24 HOURS))
 	var/year = round(days_since / 365) + 481
-	var/day_of_year = days_since % 365 + 1
+	var/day_of_year = days_since % 365
 	var/month = round(day_of_year / 28)
 	var/day_of_month = day_of_year % 28 + 1