Planned order start time calculation

Q1. In supply planning work area, planned order start date is shown and not the start time. But in production scheduling work area when we map a supply plan to production schedule, we can see the make item planned order start date and start time also. On what basis is the start time for the planned order calculated?

Q2. If we change the start date and start time of the planned order, how and where will it impact as we cannot release the planned orders from production scheduling screen.