What timezone should I use?

Pay attention to the timezones; Coinbase for example shows you the time of transaction in your timezone when you're viewing the site but once you download the data it will switch to Pacific Time, Binance will be in UTC time...etc.) If you don't have the time correct than your FIFO calculations and Short/Long term gains can be off. The default when uploading a custom CSV or entering your trades manually is to use the time zone you live in.