I would say that Harish gave you a good explanation of why it doesn't work.
Solution 1: As mentioned by Harish, add today's value to the monthly value (for allowances given on the last day of the month).
Solution 2: Create a second Time Type (ex. ZUP2 that will add yesterday's value of ZUP2 to today's value of ZUP1 in order to create today's value of ZUP2 (HRZ=LZUP2 HRS+DZUP1 ADDDBZUP2), making sure that yesterday's value of ZUP2 is not taken into account if today is the first day of the month.