PM maintenance plan: how is call date determined?

Hi, experts

     We use user-exit IPRM0002 to realize that the plan date of the due call object is delayed for 1 day if the corresponding equipment fulfill certain user status.Now the question appears.The plan date and call date of the first call object were delayed for 1 day respectively on the day when the first call object would be called. but the plan date of the first call object was delayed correctly and the call date remained the same the next day.

     Then,I traced the program, and found that the call date is calculated based on the start date.The logic is:

     Call date =  start date + (plan date - start date)*Cycle horizon / 100.

     Why is not the call date calculated like this:

     call date = plan date -  cycle * (1- cycle horizon/100).

The information about the plan:

cycle: 3 days.

call horizon: 67

     Am I right?  Could someone help me?

     Thanks.

Regards,

Vincent Tong.