How it's calculated
day number = (days in all earlier months) + day of month
Sums the lengths of every month before the chosen date, then adds the day of month, giving 1–365 (366 in a leap year). Days remaining = total − day number.
Common questions
What is the ordinal day number?
It is the position of a date within the year, so March 1 is day 60 in a common year and day 61 in a leap year.
How many days are in a leap year?
A leap year has 366 days, with February 29 added, so December 31 becomes day 366.