This is the default behavior for date/time fields in outgoing email, using the default time zone configured in Organization Profile; Account Information. However, if you are trying to include multiple or different time zones in a notification this can be undesirable. The default times zone information is appended even if the date/time field is wrapped in a DateAdd() function, so it is difficult to differentiate between the modified times.
Wrapping the date/time field, even if already modified by a DateAdd() function, in the ToString() function removes the default time zone data and resolves the issue. This will allow you to use DateAdd() to convert date/time fields to other time zones, and then manually include data about which time zone it represents.
$(ToString(DateAdd("minute",300,ScheduledStartTime))) will convert the time to five hours after the default time zone. You can then include the time zone (ex: EST, PDT) as text after the function and field.