The default time zone when scheduling messages is the time zone that is set on your account.

You can change this by clicking your name or picture in the top right corner and choosing My Account and changing the time zone under Account Preferences on the bottom right.

Type in your city and select from the drop-down list. If you do not see your city on the drop-down list, use the nearest large city.

Once you have set your time zone in My Account, this will be set as default when scheduling messages.

You can also select a custom time zone on the message draft.