after end of the month i need to start the count from 1 in a

Question

in a textbox1 i show the data like this 13-1

13 is date and 1 is 1st bill 13 is constant number and i use for loop for like (13-2) after the end of august i need 31-01 bill number i does know the last bill no after 31 august i need to start the 1-01 (september 1-((billno is)01).

for example if the billno end at 450 i show it in textbox as (31-450)(31 is date ,450 is billno)

my need is after 450 for loop show 451 .in september it show like this (1-451)...but my need is (1-01) in september