Use a date calculator to find the interval between two dates or add and subtract days, weeks and months from a starting date.
Days between two dates
Enter a start and end date to calculate the elapsed calendar-day difference. The result can be useful for project planning, deadlines and date-based comparisons.
Adding days and weeks
A day-based calculation moves the calendar by the selected number of days. A week is treated as seven days.
Adding months
Month arithmetic follows calendar months, so adding one month to a date near the end of a month can behave differently from adding a fixed number of days.
Inclusive versus exclusive counting
Some applications count both the start and end date while others count elapsed time between them. Always confirm which convention a deadline or contract uses.
Practical uses
Use the tool for planning events, study periods, subscriptions, project schedules and personal date calculations.
Limitations
Public holidays and business-day rules are jurisdiction-specific; a simple calendar-day result should not be presented as a business-day calculation unless those rules are explicitly included.
Worked example: date arithmetic
To find an elapsed interval, identify the exact start and end dates first. To add a week, move seven calendar days. To add a month, use calendar-month arithmetic rather than assuming a month always contains 30 days.
Why endpoint rules matter
A period from Monday through Friday contains five dates when both endpoints are included, but an instruction such as “five days after Monday” normally starts counting after Monday. State the convention before comparing results.
Calendar days versus working days
If weekends or holidays must be excluded, use a business-day method instead of an ordinary date difference.
Date Difference vs Date Arithmetic
Finding the difference between two dates is different from adding a period to a starting date. A date calculator should treat these as separate operations because month lengths and calendar boundaries affect the result.
Adding Days and Weeks
Adding days moves forward or backward by a fixed number of calendar days. A week represents seven days, so adding four weeks is equivalent to adding 28 days.
Adding Months Is Not the Same as Adding 30 Days
Calendar months have different lengths. A month-based calculation should therefore use calendar-month rules rather than treating every month as exactly 30 days.
Worked Example
If a date is 15 January and one month is added, the calendar result is 15 February. Adding 30 days is a different operation and can produce a different date.
Leap Years
February has 29 days in a leap year and 28 days otherwise. Date calculations that cross February must account for the actual calendar year.
Business Days Are Different
A normal calendar-day calculation includes weekends and holidays. If a deadline is defined in business days, the applicable weekend and holiday calendar must be specified separately.
Use the Tervilo Date Calculator
Use the Tervilo Date Calculator to calculate date differences or add and subtract days, weeks and months from a starting date.