Hi Deepika,
The duration of the planned order is decided by the activity times from the PDS or PPM. Which is the same durations, you maintain in ECC routing.
Since the system uses the backward scheduling and I assume you have multiple demands with the same requirement date of 20/07/2016 (I hope you mistyped 2013) - but with different quantity, the start date is calculated differently.
For example, if your PDS is set with 1 hr per EA as a production duration, then 100 EA order would need 100 hr - but 50 EA order would need only 50 hr. Then system will use the requirement date as the end date and use backward scheduling to get the start date.
Please let me know if you need any more details.
Regards,
Manimaran M.