Self life expiring date calculating wrong.

Hi Gurus,

I have one issue

We have maintained batch management for material.

In material master we have maintained total self life as 20 months. We are entering production date at GR, but system calculating expiring date as 21 months by default.Its calculating one month extra as expiring date.

How can I solve this problem , system to calculate correct expiring date?

Plz help me

Thanks

Ramesh