実は date-fns で事足りるというケースが多いよ、という話。 date-fns-tz はどういうライブラリ? date-fns-tz の README より引用。 Working with UTC or ISO date strings is easy, and so is working with JS dates when all times are displayed in a user's local time in the browser. The difficulty comes when working with another time zone's local time…