What is the IST to GMT Converter?
This tool converts a clock time in Indian Standard Time (IST) to Greenwich Mean Time (GMT). IST is fixed at UTC+5:30 year-round (India observes no daylight saving), while GMT is UTC+0. The difference is therefore a constant 5 hours and 30 minutes, with IST always ahead of GMT.
How to use it
Enter the hour (0–23) and minute (0–59) of your IST time. The converter subtracts 5 hours 30 minutes and shows the matching GMT time. If subtracting pushes the time before midnight, the result notes that GMT falls on the previous calendar day.
The formula explained
The math works in minutes since midnight. Convert IST to minutes: \(\text{IST\_total} = \text{hour} \times 60 + \text{minute}\). Subtract 330 minutes (5 h 30 m): \(\text{GMT\_total} = (\text{IST\_total} - 330) \bmod 1440\). The modulo by 1440 (minutes in a day) handles day wrap-around. Then GMT hour = GMT_total ÷ 60 and GMT minute = GMT_total mod 60.
$$\text{GMT} = \Big(\left(60 \cdot \text{Hour (IST)} + \text{Minute (IST)}\right) - 330 \bmod 1440\Big)$$
$$\begin{gathered} \text{GMT}_{\min} = \big((I - 330)\bmod 1440 + 1440\big)\bmod 1440 \\[1.5em] \text{where}\quad \left\{ \begin{aligned} I &= 60 \cdot \text{Hour (IST)} + \text{Minute (IST)} \\ \text{GMT Hour} &= \left\lfloor \tfrac{\text{GMT}_{\min}}{60} \right\rfloor \\ \text{GMT Minute} &= \text{GMT}_{\min}\bmod 60 \end{aligned} \right. \end{gathered}$$
Worked example
Suppose it is 14:45 IST. IST_total = \(14 \times 60 + 45 = 885\) minutes. GMT_total = \(885 - 330 = 555\) minutes. \(555 \div 60 = 9\) with remainder 15, so the GMT time is 09:15, same day.
FAQ
Is GMT the same as UTC? For everyday timekeeping they are effectively identical; both sit at offset +0. GMT is a time zone while UTC is a time standard.
Does India use daylight saving time? No. IST stays at UTC+5:30 all year, so the 5:30 offset to GMT never changes.
What if my IST time is early morning? Times before 05:30 IST roll back to the previous day in GMT — for example 02:00 IST is 20:30 GMT the day before.
