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 clicking My Account.
You can change your time zone under Account Preferences on the bottom right.
Type in your city and select from our drop-down list. If you do not see your city on the drop-down list please try using the nearest large city in your time zone.
Once you have set your time zone in My Account this will be set as default when scheduling messages. If you do not want to use your default time zone you can also select Custom and select a time zone there.
You can find a list of valid time zones here.