A nontrivial amount of my time is wasted responding to emails with "3pm pacific or mountain?". Asking "does 23:00 work for you?" avoids the variable.
This is twice as bad if you ever have to work with anyone in Arizona during the summer, as they already skip DST.
Fortunately calendaring software does a decent job abstracting the insanity away, but that doesn't help much with the process of scheduling things across companies, as you can't see their calendars. I suppose within a company that has offices in multiple timezones it's less painful.