Sunil,
You need to make a couple of changes.
1. Reinitialize your SNP planning area and create time series objects with 3 months in past and required number of periods into future.
- Setup a background job to extend the time series in same pattern periodically (use t.code /SAPAPO/TSINIT). For instance, assume initial setup is from 6/2013 to 8/2014 (i.e. 3months in past and 12 months in future - 15 months horizon). Next month the time series need to be extended by one month from 7/2013 to 9/2014 (i.e. drop one month in the past and add one month in future).
2. Update your planning book (t.code /SAPAPO/SDP8B) to remove defined time bucket profile for the past in data view SNP_BATCH.
Thanks,
Rajesh