Hi iamjmw,
As per your description, you just click on cell>go to data tab>click sort>select ascending order to sort your dates correctly.
For reference: Sort data in a PivotTable
If you still cannot be able to correct sort order, and if it is convenient for you, could you please share with us your Excel file and I will help you to sort in proper order. You can provide the requested files in private message. Please click here to access your PM.
I appreciate your time and understanding.
Best Regards
Waqas Muhammad