The built-in Timers return to their preset time when the Timers' Start and Run condition goes to logic 0 (OFF). This feature prevents you from accumulating Timer times.
If you want to measure accumulated time, you must build the Ladder logic to do this.
According to the above example:
MI 5 - Accumulated Timer value.
MI 6 - Accumulated Timer preset value.
MB 4 - Reset bit.
MB 1 - Start and Run Condition bit
Use SB 3 - 1 second pulse to advance the time count.