Using function module LAST_DAY_IN_PERIOD_GET
You can use the fiúnction modules FIRST_DAY_IN_PERIOD_GET and  
LAST_DAY_IN_PERIOD_GET to find the first/last day of a periode.
Doing it yourself
parameters: s_year(4) type n default '2000',
                     s_per(2) type n default '01'.
data: my_date like sy-datum.
start-of-selection.
  if s_per = '12'.
    concatenate s_year s_per '31' into my_date.
  else.
    s_per = s_per + 1.
    concatenate s_year s_per '01' into my_date.
    my_date = my_date - 1.
  endif.
  write:/ s_aar, s_per.
  write:/ my_date.
