How to allow various time formats into a field (HH:MM, hh:mm, HHMM or HMM) but always output HH:MM?

Thank you for your thoughts on the matter. I know that some scripting is needed. 

I've written a validation script and attached it to the "Properites >Validate" tab, which allows HH:MM, HHMM, hh:mm, or hhmm
But I can't figure out how to format the output to always be HH:MM

For this second part, I thought maybe a script in "Properites > Calculation"  or "Properites > Format" to do the following
(1) get field value
(2) strip out non digits
(3) set field to date format of HH:MM

For example
var enteredTime = getField("TimeField").value;
var numberPattern = /\d+/g;
var extractNum = enteredTime.match( numberPattern );
event.value = (util.printd("HH:MM", extractNum));

But it does not work. I am not familiar with JavaScript or Regular Expressions, so maybe I'm doing something wrong. Or maybe I'm not using the "util.printd" function properly. I'm lost.

Yes, but you'll need to write a custom validation (and possibly also keystroke and format) script to do it.