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Using that method, you can't and still have it be a time field. The layout format specifies the format displayed, data entry format is controlled by the system settings on your computer (locality) and what you select in File >> File Options on the Text tab.
Typically, you will want to create a text field that you allow the "editing" in. Then have your actual date be a calculation, or set the field based on the value of the edit field.
You can also use the OnValidateObject trigger to perform a script that first confirms that the data has been entered into a valid format that you want and then converts the entered data into the value and data type needed for entry into the timestamp field. This trigger will trip and run the script before the data type driven validation kicks in and complains that you've entered data with an incorrect format.
I've used this trick, for example, to set up a date field where I enter only the month and the year with the trigger controlled script then providing a 1 for the day in order to enter a valid date, but formatted to show month and year only as I had no need to specify the day in that special case.