EST to PST

Enter a time in US Eastern Time and read it in US Pacific Time. PST is usually 3 hours behind of EST.

Whether the gap holds all year

The gap holds at 3 hours behind all year, because US Eastern Time and US Pacific Time either change their clocks together or not at all — which makes this one of the few conversions you can safely memorise.

Which zones these letters mean

EST resolves to US Eastern Time and PST to US Pacific Time. Neither is ambiguous, so a time converted here means one thing only.

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.