Quick Answer: Date maths rests on a few formulas: days between dates equals end serial number minus start serial number, a leap year is divisible by 4 (but century years need 400), and time equals hours times 60 plus minutes. Combine them to add days, count weekdays or find daily interest.
Key takeaways:
- Convert dates to a day count, then subtract.
- Leap year rule: divisible by 4, and by 400 if a century year.
- Interest by days uses principal times rate times days divided by 365.
- Weekday count equals total days minus weekend days.
- Always state whether the end day is included.
The idea behind every date formula
Computers and spreadsheets do not think in months. They give every date a serial number, which is just a count of days from a fixed starting point. Once two dates are plain numbers, finding the gap is simple subtraction.
You can use the same idea by hand. Count how many days have passed since the start of the year for each date, adjust for the year difference, and subtract. This article explains the formulas one by one and shows each with an Indian example. If you only want a quick answer, a date and time calculator does the work in a second, but knowing the formulas helps you trust the result.
Formula 1: days between two dates
The core formula is short.
Days between = End date serial – Start date serial
For inclusive counting, add 1. For months and years, you split the gap into whole years, then whole months, then leftover days.
By hand, use the day-of-year method. Day of year is the sum of full months before the date plus the day number. For 9 October 2026 it is 273 (days from January to September) plus 9, which is 282. For 1 April 2026 it is 31 + 28 + 31 = 90, plus 1, which is 91.
So the number of days from 1 April 2026 to 9 October 2026 is 282 minus 91 = 191 days. Check it: April has 29 more days after the 1st, May 31, June 30, July 31, August 31, September 30, and 9 in October. 29 + 31 + 30 + 31 + 31 + 30 + 9 = 191. It matches.
Formula 2: the leap year rule
A year is a leap year if it passes this test:
- It is divisible by 4, and
- If it is a century year (ending in 00), it must also be divisible by 400.
So 2028 is a leap year. 2100 is not, because it is a century year not divisible by 400. 2000 was a leap year because it is divisible by 400.
This matters for the Indian financial year. FY 2027-28 runs from 1 April 2027 to 31 March 2028, and it contains 29 February 2028. So it has 366 days. FY 2026-27 has 365 days.
Formula 3: adding days to a date
There is no single neat formula, but there is a clean procedure. Add the days to the day number, then subtract month lengths while the day number is larger than the current month.
New day = Start day + Days to add. While New day > month length: subtract month length and move to the next month.
Worked example 1: a 180-day FD
Sneha opens a 180-day fixed deposit on 20 January 2026. When does it mature?
- New day = 20 + 180 = 200
- Subtract January (31): 169, now in February
- Subtract February (28 in 2026): 141, now in March
- Subtract March (31): 110, now in April
- Subtract April (30): 80, now in May
- Subtract May (31): 49, now in June
- Subtract June (30): 19, now in July
The remaining 19 is less than July’s 31 days, so the maturity date is 19 July 2026. Banks may adjust for a holiday, so check your deposit receipt for the final date.
Key takeaway: To add days, keep subtracting month lengths until what is left fits inside a month. The leftover number is the day.
Formula 4: interest for a number of days
Many Indian loans and overdrafts charge interest by the day. The simple formula is:
Interest = Principal x Annual rate x Days / 365
Worked example 2: 45 days of interest
Karan borrows Rs. 2,00,000 for 45 days at 9% a year simple interest.
- Annual interest = 2,00,000 x 0.09 = Rs. 18,000
- Daily interest = 18,000 / 365 = Rs. 49.32 (rounded)
- For 45 days = 18,000 x 45 / 365 = 8,10,000 / 365 = Rs. 2,219.18
Karan pays about Rs. 2,219 in interest. Some lenders use 360 days, which would give Rs. 2,250. The difference looks small, but it shows why the day-count basis in your loan agreement matters. To see how interest grows when it compounds over years, try our time value of money calculator.
Formula 5: working days between two dates
Business days are total days minus weekend days and holidays.
Working days = Total days – Weekend days – Holidays
Every full week has 5 working days. So count full weeks first, multiply by 5, then handle the leftover days one by one.
Worked example 3: a project window
A team gets a task on Monday, 12 October 2026, and must finish on Friday, 23 October 2026. Count both days.
- Total days inclusive: 12 to 23 October = 12 days
- Weeks: that is 1 full week (12 to 18 October) plus 5 days (19 to 23 October)
- Weekend days: 2 (17 and 18 October)
- Working days = 12 – 2 = 10
If the office is closed on 20 October for a local holiday, the working days fall to 9. Holidays differ by state, so always check your own calendar.
Formula 6: time and duration
For time within a day, use minutes.
Total minutes = Hours x 60 + Minutes
Duration = End minutes – Start minutes (add 1,440 if the end is on the next day)
For time zones, use the offset from UTC:
Local time = UTC + Offset
IST has an offset of +5:30. Singapore is +8:00. So Singapore is 2 hours 30 minutes ahead of IST. When it is 10:00 AM in India, it is 12:30 PM in Singapore.
Formula 7: whole months between two dates
Loan tenures, rent agreements and SIP plans are counted in months, not days. The formula is simple.
Months = (End year – Start year) x 12 + (End month – Start month)
If the end day is smaller than the start day, subtract 1, because the last month is not complete yet.
Take a home loan whose first EMI date is 15 January 2026. How many months have passed by 15 October 2026? That is (2026 – 2026) x 12 + (10 – 1) = 9 months. The end day (15) is not smaller than the start day (15), so no subtraction is needed. In days, this is 31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 = 273 days, which you can confirm by adding each month in turn.
Now change the end date to 10 October 2026. The same formula gives 9 months, but the end day (10) is smaller than 15, so the answer is 8 complete months and 25 days. This small step avoids the common error of treating a part month as a full one.
How spreadsheets use the same ideas
Spreadsheet programs store each date as a serial number and show it as a date only for display. That is why subtracting one cell from another gives days, and why adding 30 to a date cell moves it forward by 30 days. Time is stored as a fraction of a day, so 12 noon is 0.5 and 6 PM is 0.75.
Knowing this helps you debug a sheet. If a result shows as a strange number such as 45,000, the cell is simply formatted as a number instead of a date. Change the format and the date appears. The formulas in this article are the same ones working quietly behind the scenes.
Summary table of formulas
| Need | Formula | Quick example |
|---|---|---|
| Days between | End serial – Start serial | 1 Apr to 9 Oct 2026 = 191 days |
| Leap year | Divisible by 4; century needs 400 | 2028 yes, 2100 no |
| Add days | Subtract month lengths until it fits | 20 Jan + 180 days = 19 Jul 2026 |
| Day interest | P x R x D / 365 | Rs. 2,219 on Rs. 2 lakh for 45 days |
| Working days | Total – weekends – holidays | 12 Oct to 23 Oct 2026 = 10 days |
| Time zone | UTC + offset | 10:00 AM IST = 12:30 PM Singapore |
Benefits of learning the formulas
- You can detect wrong dates on bills, notices and agreements.
- You can estimate interest on short-term borrowing before you accept an offer.
- You gain confidence when planning tax payments such as advance tax on 15 June, 15 September, 15 December and 15 March.
- You understand how spreadsheets and apps reach their answers.
- You can explain your calculation to a bank, an employer or a chartered accountant.
Challenges and limitations
Formulas assume clean rules, but real life is untidy. Banks may use 360 or 365 days. Some contracts count the end day, some do not. A due date that falls on a Sunday or a bank holiday may move to the next working day, and the rule for that differs by document.
Leap years are another trap. A formula that uses 365 for every year will be off by one day in a year with 29 February. Long ranges need extra care. Finally, formulas know nothing about holidays or local customs, so you must add those yourself.
Common mistakes
- Using 30 days for each month. It is quick but wrong, and the error grows with length.
- Dividing by 365 when the contract says 360. Read the day-count basis first.
- Skipping the leap day. Check whether 29 February falls inside your range.
- Counting the start and end day twice. Decide on one rule and stick to it.
- Forgetting that weekends differ. Some Indian offices work a six-day week.
- Ignoring holidays on a due date. If a due date is a holiday, check the portal rule on whether it moves.
Best practices
- Write the formula and the numbers on paper before you calculate.
- State the counting rule (inclusive or exclusive) with the final answer.
- Verify the result with a second method, such as counting forward.
- Check official due dates on incometax.gov.in or gst.gov.in.
- Keep a state-wise holiday list handy when counting working days.
- Use a calculator for long ranges and keep the formulas for sense checks.
Conclusion
Date and time formulas are small tools that fit together. Serial numbers give you day counts, the leap year rule keeps you accurate, subtraction of month lengths lets you add days, and minutes make time simple. Try the three examples again with different dates. With a little practice, you will spot errors before they reach your tax filing, your loan statement or your offer letter.
- Try the free Date and Time Calculator →
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Frequently Asked Questions
What is the formula to find days between two dates?
Days between equals the end date serial number minus the start date serial number. Add 1 if you want to count both days. By hand, use the day-of-year method and adjust for years.
How do I know if a year is a leap year?
A year is a leap year if it is divisible by 4. Century years such as 2100 must also be divisible by 400. So 2028 is a leap year, but 2100 is not.
Should I use 365 or 360 days for interest?
Use whatever your loan or deposit agreement states. Most Indian retail products use 365 days, but some money market products use 360. The choice changes the interest slightly.
How do I count working days between two dates?
Count the total days, subtract the weekend days, and then subtract public holidays that fall in the range. Each full week gives 5 working days.
How many days are in FY 2027-28?
FY 2027-28 has 366 days because it includes 29 February 2028. The Indian financial year always starts on 1 April and ends on 31 March.
