Hi Shraddha,
Its not clear if you want SNP heuristic planning to consider the GR processing time or writing a Zprogram to consider GR processing time.
If you want GR processing time to be considered in SNP planning, then please note as a standard functionality SNP heuristic considers the GR processing time maintained in the product master.
If writing a Zprogram and having issue with time stream field in the said FM, you may refer to time stream table /sapapo/tcalperi, wherein you can take the correct format for the time stream which you can use in this function module.
Hope this helps!
Regards,
Umesh