Convert a time between two zones

Pick a time and two places. The result names which zone each abbreviation was resolved to, and says whether the gap between them holds all year or shifts when one end changes its clocks.

Abbreviations are ambiguous and we say so

CST is US Central Time and it is also China Standard Time, fourteen hours apart. IST is India, Ireland and Israel. A converter that silently picks one and prints a number is how meetings get missed, so every result here names the zone it used and offers the other reading.

The gap between two zones is not one number

The United States and the European Union change their clocks on different dates, so for about four weeks a year the usual difference is an hour out. Each conversion page states the range across the year rather than a single figure that is quietly wrong twice a year.

Questions

What does CST actually mean?

It depends who wrote it, and that is the problem. CST is US Central Time and it is also China Standard Time — fourteen hours apart. IST is India, Ireland and Israel. Every conversion page here names the zone it resolved an abbreviation to, rather than quietly picking one.

Why does the difference between two time zones change during the year?

Because countries change their clocks on different dates. The United States moves in March and November, the European Union in the last weekends of March and October, and the southern hemisphere moves the opposite way entirely. For a few weeks each year the usual gap is an hour out, which is when most mis-scheduled calls happen.

Why is Nepal not a whole number of hours?

Because time zones are political rather than geometric. Nepal is UTC+5:45, India UTC+5:30, and the Chatham Islands UTC+12:45. Any converter built on whole hours is wrong for entire countries, which is why nothing here rounds.