Date Calculators
Four free date tools: add or subtract days from a date, count down to a target date, count working days between two dates, and find the week number of any date in both ISO 8601 and US conventions.
Choose a date calculator
Add or Subtract Days, Weeks, Months from a Date
Pick a base date, enter how many days, weeks, months or years to add (use negative numbers to subtract), get the result instantly.
Countdown Calculator — Days, Hours, Minutes Until Any Date
Enter a target date — see exact days, hours, minutes and seconds remaining (or elapsed if the date is in the past).
Working Days Calculator — Count Business Days Between Dates
Enter two dates — get the count of weekdays (Monday through Friday) between them, inclusive. National holidays are not subtracted.
Week Number Calculator — ISO 8601 and US Convention
Pick any date — see its week number in both ISO 8601 (Monday-start, used in EU and most of the world) and US convention (Sunday-start).
FAQ
What is the difference between ISO 8601 and US week numbering?
ISO 8601 weeks start on Monday and week 1 is the week containing the first Thursday of the year. US convention weeks start on Sunday and week 1 is the week containing January 1. The two systems often disagree by one week near year boundaries — important for payroll, scheduling and international reporting.
What counts as a 'working day'?
In this calculator, a working day is any Monday through Friday. Public holidays are NOT subtracted because holiday calendars vary by country, region and even employer. If you need a holiday-aware count, subtract the relevant holidays from the result for your jurisdiction.
Do these calculators handle leap years?
Yes. All four calculators use Go's standard time package, which correctly handles leap years (year divisible by 4, except century years not divisible by 400). February 29 is a valid date in leap years and rolls over to March 1 in non-leap years for date arithmetic.
What time zone is used?
All calculations use UTC midnight for date math. Since these tools work with calendar dates (not timestamps with time-of-day), time zone differences only matter near the day boundary. For most use cases the result is identical regardless of your local time zone.
Why would I need a countdown calculator?
Countdown calculators are useful for project deadlines, contract milestones, event planning, exam dates, travel preparation, and any situation where you need to know exactly how much time remains. The seconds-level precision helps with last-minute decisions.
Does this work for non-Gregorian calendars?
No. All four calculators use the proleptic Gregorian calendar (the international civil standard). For Hebrew, Islamic, Persian, Buddhist or other calendar systems, you would need a specialized calendar conversion tool.