DIY Cost Tools
Day of Week Calculator
TIME & DATE CALCULATOR
Find the weekday, day of year, ISO week number, and days remaining in the year for any date.
How it works
The selected calendar date is created in local time. The calculator counts its ordinal day, computes the ISO week, and compares it with the first day of the next year.
Formula
Day of year = whole days since January 1 + 1. ISO weeks begin Monday, and week 1 is the week containing January 4.
Worked example
September 29, 2026 is Tuesday, day 272 of the year, in ISO week 40, with 93 days remaining after that date.
Important limits
- Historical calendar systems and regional adoption dates are not modeled.
- ISO week years can differ from calendar years near January 1 and December 31.
- The weekday follows the proleptic Gregorian calendar used by the browser.
Frequently asked questions
Why can late December be ISO week 1?
ISO week numbering assigns a week to the year containing that week’s Thursday.
Does days remaining include the selected date?
No. It counts full calendar days after the selected date through December 31.
Sources and review notes
The calculator follows standard Gregorian date arithmetic and ISO 8601 week-numbering rules. Last reviewed September 29, 2026.