The Half-Hour Gap: Why Indian Standard Time is GMT+5:30?

Alright kids, today we’re taking a trip down memory lane to high school Geography. But before we do that, a quick preamble.

Daylight Savings. What a wild concept! Surprising to me 15 years ago when I came to this country. Straight up shocker to my mother-in-law when she first heard about it 5 years ago.

“What do you mean you change the time?” she said astonished at this game of Doctor Strange that all of America played collectively twice a year.

I could also never remember when was Standard Time versus Daylight Time so Eastern Standard Time (EST) and Eastern Daylight Time (EDT) went out of my vocabulary to make way for the simpler, Eastern Time (ET) instead. I have since learned that America is really on Standard time for only 4 months out of 12 so…is that really “standard”? But I’m neither Doctor Who nor Doctor Strange and I don’t make the rules. 🤷🏽‍♂️

“But Harjas, why do you care so much?”

Good question, dude. Well, my family lives in India so this totally shakes up our coordination of how and when we communicate. During “standard” times the difference is 10.5 hours, during daylight hours we go to 9.5 hours - assuming a continued East Coast residency.

When I lived in Seattle, the time difference was, of course, 13.5 hours and 12.5 hours respectively. My brother lives in California now so imagine the double math my parents need to be doing to talk to their kids!

As you’ll notice in my griping, I mention that India is on this 0.5 hour offset which catches people by surprise. When I was in college and I’d tell people that the time difference is 9.5 hours, they’d say “you mean 9 hours?” and I would reply, “no, 9.5” as I tried to explain how that was even possible. So here we go. This is one of those facts from eighth grade high school Geography that lives in my head rent-free.

Let’s start with some established facts around how the Earth is mapped, then I’ll share how the time is calculated:

1. •

   The earth is mapped using imaginary vertical and horizontal divisions called longitudes and latitudes.

2. •

   There are 180 latitudes — rising from the equator at 0° up to the poles 90° north and south.

3. •

   There are 360 longitudes — starting from the Prime Meridian, 0° longitude, in the UK (Greenwich Mean Time) and extended 180° East and 180° West.

4. •

   For our purposes, we care about time, so we really only care about the longitudes. You go East of the Prime Meridian, you “gain” time (India), you go West you lose time (United States).

5. •

   Each degree of longitude represents 4 minutes of time. 1° east is +4 mins GMT, 2° east is +8 mins GMT and so on. So 1 hour = 15°.

6. •

   Time zones are determined by a line of longitude passing through a country.

7. •

   Ideally, time zones land at the top of the hour so they’re easy to remember — GMT-4 (Eastern Time), GMT-5 (Central Time), GMT-6 (Mountain Time), meaning these lines of longitude pass through parts of the US.

8. •

   For India, moving further east from GMT, Pakistan sits at 75° East — GMT+5. If India wanted a round 6 hours, that would be 90° East (90/15 = 6).

9. •

   The 90° East longitude misses a majority of the Indian subcontinent. You actually end up in Bangladesh. Stay with me now, we’re almost there!

10. •

    So you split the difference at 82.5° East (75 + (90-75)/2). That longitude cuts just off center and covers the Indian subcontinent — 82.5 × 4 = 330 minutes, or 5.5 hours. Indian Standard Time = GMT+5:30.

11. •

    This should also tell you how fucking big the US is! India is living in half-hour increments while the continental US spans 4 hour-long time zones.

And that is why India has this half-hour offset! Despite the world clocks in all our phones, time zones are a thing immigrant kids need to worry about more than most. And my job in all this continues to be to let my parents and mother-in-law know that time is made up and it has, once again, changed.

PS: Nepal sits at GMT+5:45 for these same reasons. Imagine the cross-continental Zoom meeting coordination that takes!