create a new maintenance period, switch to the "periods" tab. add a period.
switch to daily, weekly or monthly and enter "-2" for both hours and minutes. add the period.

notice how the negative hour is kept (displayed as "At 0-3:58"), but the negative minute ended up being subtracted from the time, reducing the hour value by one.

attempting to save such a maintenance period results in an sql error :

Error in query [INSERT INTO timeperiods (timeperiod_type,every,month,dayofweek,day,start_time,start_date,period,timeperiodid) VALUES ('2','1','256','0','2','-7320','20170803000000','82800','1')] [Out of range value for column 'start_date' at row 1]