Five time tools in one: find the duration between two times, add or subtract hours and minutes, convert IST to any major world time zone, work out timesheet hours (with optional INR pay), and switch between 12-hour and 24-hour clock format.
Key Takeaways
- All of India runs on a single time zone. Indian Standard Time (IST, UTC+5:30) applies from Gujarat to Arunachal Pradesh with no regional variation and no daylight saving shift at any time of year.
- A time zone's UTC offset can change twice a year for countries that observe Daylight Saving Time (most of the US, UK and EU), which is why a fixed IST-to-EST gap of "10.5 hours" isn't always correct — it becomes 9.5 hours during US daylight saving.
- Overnight spans need a day rollover, not a negative number. A shift from 10 PM to 6 AM isn't "-16 hours"; it's 8 hours once you correctly treat the end time as falling on the next calendar day.
What This Time Calculator Does
This is five time-related calculators in one: a duration calculator that finds the exact gap between two clock times (including overnight spans), an add/subtract tool for working out a new time after adding or removing hours and minutes, a time zone converter built around India Standard Time for scheduling calls with the US, UK, Gulf, Singapore, Australia, Japan or China, a work-hours/timesheet calculator that also computes pay when you enter an hourly rate in INR, and a quick 12-hour to 24-hour (military time) converter. It's built with India's single, no-daylight-saving time zone as the default reference point, since the most common real need here is converting IST to a client, employer, or family member's time zone abroad rather than converting between zones inside India, which doesn't apply.
Everything runs entirely in your browser. No time, date, or pay rate you enter is sent to a server, logged, or stored anywhere — close the tab and it's gone. There's no signup, no email wall, and no ads inside the calculator itself. Whether you're scheduling a call with a US client, working out how long a night shift ran, or converting a flight's departure time, switch tabs and every result updates instantly.
How to Use This Time Calculator
Every tab works the same way: pick it, fill in its fields, and the result updates instantly — there is no "Calculate" button anywhere. Here's the exact step-by-step for each of the six tabs, with a worked example so you can check your own result against a known one.
1. Time Duration
- Open the Time Duration tab (it's the default).
- Enter the Start Time.
- Enter the End Time.
- Read the gap between them below — if the end time is earlier on the clock, it's automatically treated as the next day.
2. Add/Subtract Time
- Switch to the Add/Subtract Time tab.
- Choose Add or Subtract.
- Enter the Starting Time.
- Enter the Hours and Minutes to apply.
3. Time Zone Converter
- Switch to Time Zone Converter.
- Enter the time you want to convert.
- Pick the From zone (defaults to IST) and the To zone.
- Tick the Daylight Saving checkbox only if that zone is currently observing it.
4. Work Hours
- Switch to Work Hours.
- Enter Time In and Time Out.
- Enter your Break duration in minutes.
- Optionally enter an Hourly Rate (INR) to see estimated pay.
5. 12hr ⇆ 24hr
- Switch to the 12hr ⇆ 24hr tab.
- Enter any time using your device's time picker.
- Read both the 12-hour and 24-hour formats instantly.
6. Add/Subtract Duration (D:H:M:S)
- Switch to the last tab, Add/Subtract Duration.
- Enter the First Duration in Day/Hour/Minute/Second.
- Choose Add or Subtract.
- Enter the Second Duration, or hit Clear to reset both to zero.
A date of birth or start time later than the end value isn't valid in the Duration tabs and is either rejected or auto-corrected to the next day, as noted above. Every field also accepts direct typing, so you don't have to use the picker if you'd rather type a time out.
The Time Calculation Formulas, Explained
- Duration: Duration = End Time − Start Time (add 24 hours if the result is negative, to account for an overnight span)
- Add/Subtract: New Time = Starting Time ± Duration (wrapping around a 24-hour clock and flagging a day change)
- Time Zone Conversion: Target Time = Source Time + (Target UTC Offset − Source UTC Offset)
- Work Hours: Worked Hours = (Time Out − Time In) − Break Duration
Worked example (Time Zone Conversion): It's 3:00 PM (15:00) IST. IST is UTC+5:30 and US Eastern Time in winter (EST, not daylight saving) is UTC-5:00.
- Offset difference: (-5:00) − (+5:30) = -10:30
- Target Time = 15:00 + (-10:30) = 04:30
- Result: 4:30 AM EST, the same calendar day
The same logic runs for Work Hours (a 9:30 AM to 6:15 PM shift with a 45-minute break comes to exactly 8 hours) and for Add/Subtract (adding 3 hours 30 minutes to 10:45 PM rolls over to 2:15 AM the next day).
IST Time Difference Reference Table
Standard (non-daylight-saving) time difference from India Standard Time to common destinations for NRIs and remote workers — add or subtract the "IST is ahead by" figure to convert.
| Location | UTC Offset | IST is Ahead By |
|---|---|---|
| UK (GMT, standard time) | UTC+0 | 5 hrs 30 min |
| US Eastern (EST, standard time) | UTC-5 | 10 hrs 30 min |
| US Pacific (PST, standard time) | UTC-8 | 13 hrs 30 min |
| UAE / Gulf (Dubai) | UTC+4 | 1 hr 30 min |
| Singapore | UTC+8 | -2 hrs 30 min (behind) |
| Australia Eastern (Sydney, standard) | UTC+10 | -4 hrs 30 min (behind) |
| Japan (Tokyo) | UTC+9 | -3 hrs 30 min (behind) |
Quick Definitions
Indian Standard Time (IST) is India's single official time zone, fixed at UTC+5:30 and used nationwide with no regional variation. Daylight Saving Time (DST) is a seasonal one-hour clock shift used by roughly 70 countries, mostly in North America and Europe, to make better use of evening daylight — India has never observed it.
Frequently Asked Questions
No. Despite spanning roughly two geographic hours of longitude east to west, all of India uses a single official time zone, IST (UTC+5:30), with no regional or seasonal variation.
Because the US shifts its clocks for Daylight Saving Time for about eight months of the year, while IST never changes. The IST-to-Eastern-Time gap is 10.5 hours in US winter (EST) and 9.5 hours during US Daylight Saving (EDT).
Whenever the end time is earlier on the clock than the start time, treat it as occurring on the next calendar day and add 24 hours before subtracting — the Time Duration and Work Hours tabs both do this automatically.
Military (24-hour) time counts hours from 00:00 at midnight to 23:59 just before the next midnight, with no AM/PM label needed. 12-hour time repeats 1 through 12 twice a day and relies on AM/PM to disambiguate, which is where most manual conversion mistakes happen.
The Duration and Add/Subtract tabs are built for single-day and overnight spans. For a difference spanning several calendar days, use a date-based tool like a Date Difference or Age Calculator instead.
No. Every calculation, including the optional hourly rate in the Work Hours tab, runs locally in your browser and is never transmitted to any server or saved once you close or refresh the page.