Hi @bbsin
I made a simple example about you case:

In the TextInput.Default add the following code:
00:00:00
Set the OnChange property of the Text Input box (TextInput1) to following:
TextInput1.OnChange
If(
IsMatch(TextInput1.Text, "\d{2}\:\d{2}:\d{2}"),
true ,
Notify("The time format you typed is not valid, the correct format should be (HH:MM:SS) 09:00:00 or 16:00:00",NotificationType.Error);Reset(TextInput1)
)
TextInput1_1.OnChange
If(
IsMatch(TextInput1_1.Text, "\d{2}\:\d{2}:\d{2}"),
true ,
Notify("The time format you typed is not valid, the correct format should be (HH:MM:SS) 09:00:00 or 16:00:00",NotificationType.Error);Reset(TextInput1_1)
)
Note: You should type xx:xx:xx string value within the Text Input box.
The (:) should not be missed when you type the string value.
More details about the IsMatch function, please check the following article:
IsMatch function
Then to calculate the duration put this on a label:
DateDiff(TimeValue(TextInput1.Text), TimeValue(TextInput1_1.Text), TimeUnit.Minutes ) & " minute(s)"
I hope this helps you.
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