Hello @Anonymous,
You most likely have a "Submit" button that allows the user to save the changes (via SubmitForm or Patch for example).
I would suggest to disable that button when the user selects a wtrong date. Additionally, you could color the background of the DatePicker control differently and add a message. Example:
1. Copy the following formula to the DatePicker.Fill field: If(DatePicker1.SelectedDate <= Today(), RGBA(255, 255, 255, 1), RGBA(255, 0, 0, 1))
2. Copy the following formula to the Submit button's DisplayMode field: If(DatePicker1.SelectedDate <= Today(), DisplayMode.Edit, DisplayMode.Disabled)
Similarly, you can "unhide" a label with a warning message.