Adding Days to Date in SAP APO

Hi All,

I have a requirement where in I  have to add days which is GR processing time of product to a given date.

I found FM: /SAPAPO/MSDP_SNPHEU_DAYS_ADD.

But I am getting error:

Entries for time stream <Cal> have not yet been maintained.

any idea about the error or is there any other FM?