What is the IST to CST Converter?
This tool converts a clock time in India Standard Time (IST) to Central Standard Time (CST), the standard time used in the central United States and parts of Canada and Mexico. IST is fixed at UTC+5:30, while CST is UTC−6:00. The fixed gap between them is exactly 11 hours and 30 minutes, with India always ahead.
How to use it
Enter the IST hour (0–23, 24-hour format) and the minute (0–59). The calculator subtracts 11.5 hours and displays the equivalent CST time, noting whether the result rolls back to the previous day. Note that this uses standard time (CST); during US daylight saving time the central zone observes CDT (UTC−5), which is only 10.5 hours behind IST.
The formula explained
Because IST = UTC+5:30 and CST = UTC−6:00, the offset between them is (+5:30) − (−6:00) = 11:30. To go from IST to CST you subtract this amount: CST = IST − 11:30. If the subtraction takes the time before midnight, the date moves back by one day.
$$\text{CST} = \left(\left(\left(60\cdot\text{IST Hour} + \text{IST Minute}\right) - 690\right) \bmod 1440\right) \;\text{minutes}$$The full conversion, accounting for day wrap-around, is:
$$\begin{gathered} \text{CST}_{\text{total}} = \left(\left(T - 690\right) \bmod 1440 + 1440\right) \bmod 1440 \\[1.5em] \text{where}\quad \left\{ \begin{aligned} T &= 60\cdot\text{IST Hour} + \text{IST Minute} \\ \text{CST Hour} &= \left\lfloor \text{CST}_{\text{total}} / 60 \right\rfloor \\ \text{CST Minute} &= \text{CST}_{\text{total}} \bmod 60 \end{aligned} \right. \end{gathered}$$
Worked example
Suppose it is 10:00 IST. Subtract 11 hours 30 minutes: \(10\!:\!00 - 11\!:\!30 = -1\!:\!30\), which wraps to 22:30 CST the previous day. For 20:00 IST: \(20\!:\!00 - 11\!:\!30 = 08\!:\!30\) CST the same day.
FAQ
Why 11.5 hours and not a whole number? India uses a half-hour offset (UTC+5:30), so the difference with most US zones includes a 30-minute component.
Does this handle daylight saving? No — it converts to standard CST (UTC−6). When the US is on CDT (summer), use a 10.5-hour difference instead.
What does "previous day" mean? Subtracting the offset can push the CST time before midnight, so the calendar date is one day earlier than the IST date.
