Why would December 31st ever be day 366 instead of 365?

In a leap year, February has 29 days instead of 28, shifting every later date by one.

Is this the same as a "Julian date"?

Informally yes, though "Julian date" is also used for other, unrelated date systems in astronomy — this tool specifically gives the ordinal day-of-year number.

Day of Year Calculator

Find a date's ordinal day number within its year (1–365 or 366).

Date

Day 256 of the year

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How it works

1
Pick a date

Use the date picker.
2
See its day-of-year number

January 1st is always day 1; December 31st is day 365 or 366.

FAQ

Why would December 31st ever be day 366 instead of 365?: In a leap year, February has 29 days instead of 28, shifting every later date by one.
Is this the same as a "Julian date"?: Informally yes, though "Julian date" is also used for other, unrelated date systems in astronomy — this tool specifically gives the ordinal day-of-year number.