Find the hour difference between two UTC offsets.
A time zone is an offset from Coordinated Universal Time, and the difference between two places is the difference between their offsets. Most are whole hours, but a number are not: India runs at UTC+5:30, Nepal at UTC+5:45, and parts of Australia at UTC+9:30. Assuming whole-hour offsets is a common source of scheduling errors with South Asia in particular.
Daylight saving is the larger complication, because it makes the offset a function of the date rather than a fixed property of the place. Two cities can be five hours apart for part of the year and four for another, and because the northern and southern hemispheres shift in opposite directions, the gap between them can swing by two hours. Transition dates differ by country and change from time to time, and many countries — including India — do not observe it at all. For anything that matters, schedule against a specific date rather than a remembered offset, and state the zone explicitly rather than a bare clock time.
India uses a single national time based on a meridian near the centre of the country, which works out at five and a half hours ahead of UTC. Several other places also use half-hour or quarter-hour offsets, Nepal at 5:45 among them.
Because of daylight saving. When one observes it and the other does not, or when they switch on different dates, the gap changes. Northern and southern hemisphere shifts run in opposite directions, so the swing can be two hours.
UTC is the modern atomic time standard; GMT is a time zone that happens to equal UTC when daylight saving is not in effect. In casual use they are interchangeable, but UTC is the correct reference for calculation.
Specify the date, the time and the zone together, and confirm against that specific date rather than a remembered offset. Daylight saving transitions mean an offset valid in January may be wrong in July.