I need to subtract N days from a Date-Time string THEN set the time to 07:00:00

This works correctly, but I do not know to remove the Time portion and append the fixed time of 07:00:00 o the result and assign it to the startdate variable.

startdate = startdate.toTime("yyyy-MM-dd'T'HH:mm:ss").subDay(56).toString("yyyy-MM-dd HH:mm:ss");