Calculating the Difference Between Two Dates

Date differences come up constantly: How many days until a deadline? How long has a project been running? How many hours remain in a billing period? The right approach depends on what unit you need the answer in.

The simplest date difference is a plain day count. The formula is always:

Example: March 1 to March 25

When crossing months, count the remaining days in the first month, add full middle months, then add the days in the final month.

Example: January 20 to March 5

Once you have the number of days, converting to hours is straightforward — but watch out for whether you mean calendar hours or working hours.

Example: 44 calendar days

For project planning, billing, and SLAs, working days matter more than calendar days. To count working days manually:

• 1.Find the total calendar days between the two dates.
• 2.Subtract the number of full weekends (every 7 days contains 2 weekend days).
• 3.Subtract any public holidays that fall within the range.

For longer ranges, you may want the answer in months or years. The key thing to remember is that months have different lengths, so this is always an approximation unless you count calendar months precisely.

For exact calendar months (e.g. "how many full months from Jan 15 to Apr 15"), count the months directly: Jan→Feb = 1, Feb→Mar = 2, Mar→Apr = 3. Answer: 3 months exactly.

The reverse problem — "what date is 90 days from today?" — is solved by adding the duration to the start date.

Example: 90 days from March 1

TCalc.it is optimised for duration arithmetic — adding and subtracting hours and minutes. For a project that ran 3h 20m on Monday, 5h 10m on Tuesday, and 2h 45m on Wednesday, simply enter those durations and sum them:

Then multiply by your hourly rate or convert to decimal hours for reporting (11h 15m = 11.25h).