Ok. Than is that your problem I am afraid. "Fix" Excel and sorting will be possible.
I do not know how many items are in the Excel table, but perhaps - I let that testing up to you - it is also possible to create a collection using the SortByColumns function and the AddColumns function and the DateValue function on the Excel table. This collection is then used the property "Items".
ClearCollect(newTable, SortByColumns(AddColumns(DailyCheck, "newDateCompleted", DateValue(DateCompleted)), "newDateCompleted")
Please share your results with us.
Rick