The Format Date/Time String function returns the date time string since 12:00 AM, Friday, January 4, 1901 Universal Time (UT: I think that used to be known as GMT). The time is displayed for your time zone. 0 seconds = 12:00 AM UT = 19:00 (or 7 PM) Eastern Standard Time. If you change time zone (or go to Daylight Savings Time), time 0 will change for you. Format Date/Time String isn't really intended to be used as a timer.
You may want to use Get Date/Time in Seconds. Call it when you want to start your timer, then call it again when you want to display the time. Subtract the two values and parse it into H:M:S yourself.