Make Ontario a TDY location in DTS. Example:
Home Base = Depart 13 sep
TDY Location 1 = Edwards (assuming you fly into Ontario, but go to Edwards that day), arrive 13 sep, depart 19 sep
TDY Location 2 = Ontario, arrive 19 Sep, depart 20 Sep (early flight out the 20th).
Home Base = Arrive 20 Sep
Problem solved since per diem is tied to TDY location. Never had a problem once using this method.