> In summer, London is not UTC+0. They mean "UTC+0 ignoring DST", but that is not useful

This is how 99% of people interpret it

It's not ambiguous as you imply.

Summer time is not the default time

I don't know enough about the India case to know how/why it's wrong though

Tell that to Ireland where we observe Irish Standard Time during the summer (and GMT during the winter).

Tell what? Nobody cares, because everybody knows what is being said

Your TZ doesn't change between summer and winter. What changes is the shift

I literally didn't see anybody getting confused with this in any country (yes, including Ireland) with summer time.

But some people think they're too smart when they nitpick about minor issues

	▲	rlpb 3 days ago \| parent [-]
		> Your TZ doesn't change between summer and winter. What changes is the shift My TZ is GMT in winter and BST in summer. I am not in GMT in summer. GMT continues to exist in summer, doesn't shift but my clock doesn't follow it. The UTC "shift" changes indeed. When I am DST-shifted, calling me "UTC" is absolutely wrong. The practical issue is that people still use "UTC" and "GMT" interchangeably, which is roughly correct anyway since they remain the same in practice. But then during summer when someone says GMT I don't know if they actually mean BST (they mean my local time) or UTC (they mean the global point of reference). That ambiguity only arises because Outlook (and you, apparently) conflate GMT and BST. It's far more of a problem for those actually living in a UTC-adjacent time zone (do you?), especially because being only one hour off, usually both options seem equally likely in context.