Hello @nagestiada ,
you can refer the below formula:
Collect(EmptyDates,Filter(YourDataSource, YourDateColumn=Blank()));
ForAll(EmptyDates, Patch(YourDataSource,ThisRecord,{YourDateColumn:Date(2020,12,25) + Time(9,30,0)}))
If you use Datepicker you can directly use as below
Collect(EmptyDates,Filter(YourDataSource, YourDateColumn=Blank()));
ForAll(EmptyDates, Patch(YourDataSource,ThisRecord,{YourDateColumn:YourDatePicker.selectedDate}))
I referred this link by @mdevaney's article for Date time column Patching.