Hello
To achieve Basic Start date of the Order should not copied from the Notification. But it should be as current date when Order created.User Exit can be used IWO10009.
Additionally ask ABAPer to check any Implicit or Explicit enhance point in work Order Header Include to write some come to keep those dates as it is when work order is created via IW21,IW22,IW28 and all transaction codes from where work order created .
BR
Rakesh