Here's the shortlist of horrors I've had to deal with in my career:

• Mixed US/ROW short date formats - DD/MM/YY, MM/DD/YY
• mixed timezones in the same column
• the wrong timezone (marked as PDT but actually UTC, or sometimes the other way around)
• clock drift
• timezones again...because timezones suck
• historical timezones
• NTP configurations

Things I've read about but haven't needed to deal with personally:

• leap seconds
• clock slew vs skip
• hardware clocks
• PTP

The one thing I really don't care about is relativity