about a table problem by use SM30

HI !

I have a custom table, the fields have 2 col are begda and endda , when I input one record, begda = 01.01.2007 and endda = 31.12.9999 , at this ,when I input the second record

example, begda = 12.12.2007 , endda = 31.12.9999 , and the other cols are same to first record,

this case ,I want the first record endda field can auto change as '11.12.2007' , and let these 2 records not clash, how can I to modify ?can you help me ? thanks !