https://userapps.support.sap.com/sap/support/knowledge/en/1518552
1518552 - The SchedulingInfo.BeginDate returns a different value in XI 3.1 SP3 than it did in prior...
In XI 3.1 SP2 the SchedulingInfo.BeginDate on a recurring object returned the next time the instance would run. This is from the SI_StartTime property.