How they calculate the gst time and ist time?

Question

about time ratio in country to country

Answer 1

First is GMT (and not GST)

Time through Greenwich in UK is taken as the reference point, say 00.00 hrs. The globe is divided into 360 longitude degrees. For one full rotation it takes 24 hours or 1440 minutes. So for each degree it amounts to 1440/360=4 mins. India lies at 82.5 degree longitude east of UK or Greenwich. That means the difference is 82.5 times 4 minutes or 330 minutes or 5 hrs and 30 minutes. Thus when it is 00.00 hrs in UK(GMT) it is 05.30 hours in India, i.e. it is added for all locations east of Greenwich. For west of UK it will be deducted. Similarly for each country with the longitude detail known, it can be calculated. ( hope my memory of words and data are correct as I complted my schooling 35 years back!)