I encountered one quirky little issue when updating some of my contacts birthdays in the native Contact calendar. Here is what would happen to each and every contact:
When you enter a birthday let’s say the 8th of January, it would immediately show the 7th of January upon saving. Having traveled around in the past I knew something was out of whack with timezones but did not know where to start troubleshooting, the first obvious start was Google no relevant solution was out there.
After, about 30 minutes I discovered a magic setting in the macOS Calendar application called Turn on time zone support which is visible under Preferences -> Advanced
Once this is enabled you should be golden. At least this is what worked for me on macOS High Sierra.