@Anonymous
Well...depends. You originally stated that you had a TextInput control where users were entering the value (not really recommended).
I would recommend a DatePicker and three dropdowns.
DatePicker is pretty obvious - let's call it dpDate
Next, add a Dropdown and call it ddHour. Set the Items property to Sequence(12)
Add another Dropdown and call it ddMinute. Set the Items property to : ForAll(Sequence(60, 0), Text(Value, "00"))
Add another Dropdown and call it ddAmPm. Set the Items property to : ["AM", "PM"]
Now, if you want the date and time to write to your SharePoint list, then use this in your formula:
DateTimeValue(Text(dpDate.SelectedDate, ShortDate) & " " & ddHour.Selected.Value & ":" & ddMinute.Selected.Value & " " & ddAmPm.Selected.Value)
This will return a complete date and time value that can be written to the datasource.
Make sure your SharePoint date column includes times, then implement this into your app.
Get this going and then let me know when you get to the "displaying a previously entered date" part of your scenario as you will need to do some setting of Default properties to set the dropdowns properly based on the prior entered value.