Hi @Anonymous ,
Could you please share more of your requirements?
Do you want to compare the two data values when you click submit button?
I think you can use datediff function to help you achieve that. I test as next:
Data Source:

App Test:
Add one label control,and rename it as Label_Message.
Icon1(Submit).OnSelect=If(DateDiff(DataCardValue5.SelectedDate, DataCardValue4.SelectedDate )>0,SubmitForm(Form1))
Label_Message.Text="You input DateEnd<DateStart,please input again!"
Label_Message.Visible=DateDiff(DataCardValue5.SelectedDate, DataCardValue4.SelectedDate )<0
Label_Message.Color=Red

Hope this could be helpful.
Best Regards.
Yumia