help with powershell

Hi,

I'm new with powershell. I have a simple txt file with one line and numerisk value.

I need a script that runs every day and change this value with -1.

exp. The value is always in line 1

old value - 20140115

shuld be after script run: 20140114

so I need replace the "old value" with "old value-1)

Robert