Cross-timezone scheduling fails in predictable ways. Someone writes "3pm" with no zone. Someone else does the conversion in their head and gets it wrong by an hour. Daylight saving shifts on different dates in different countries, so a recurring call drifts twice a year.
Never write a bare time
"Thursday 3pm" is not a time. It is a time in someone's head. Always store and send times with an explicit zone, and let each person read them in their own.
Let the tool do the conversion
Humans are bad at mental timezone math under mild social pressure. Software is not. Every time in Musterd is stored in UTC and rendered in the viewer's local zone, with the zone printed on screen so nobody has to guess.
Watch the daylight saving gaps
The US and Europe change clocks on different weekends. For a few weeks each year, the usual offset is wrong. If you are scheduling in that window, double-check the date, not just the time.
A time that is unambiguous on screen removes an entire class of missed meetings.