Hi Bob,
This may be an issue with the values and the corresponding date time – actually the grouping.
You can try to add Sorting in the Pivot >> Data Groupings >> Categories >> Sorting. Something like:
Expression: =Fields!Time_Stamp.Value
Direction: Ascending
Let us know if this helps.
PS - there was no screenshot with your post, please try to attach it again.
Best Regards,
Nevron Support Team