Date Calculator
About this tool
Adds or subtracts days, weeks, months or years from a start date and shows the date and weekday. 100 days after 8 Oct 2026 is Sat 16 Jan 2027. Use a negative number such as -30 to go back.
With "Count start date as day 1", the start date itself is day 1 — used for 100-day anniversaries — giving 15 Jan 2027 in the same example.
Months and years never overflow the month end: 31 Jan plus one month is 28 Feb (29 in leap years). For the number of days between two dates, use the D-Day Calculator.
FAQ
How do I count 100 days?
For anniversaries counted from day 1, turn on "Count start date as day 1"; otherwise the next day is day 1.
What is 31 Jan plus one month?
February has no 31st, so it becomes the last day of February.
Days between two dates?
Use the D-Day Calculator.