This formula is supposed to subtract one day every day. It works perfectly to
30 days, and then on the 31st day, it resets to zero. It needs to go
indefinitely, and I'm at a complete loss. Any suggestions?

=R19-DATE(YEAR(R19),MONTH(R19)-IF(DAY(R19)<DAY(L19),1,0),DAY(L19))&"days"

Excel 2003