Personally, we had a field that was creating confusion because people kept selecting the date, not changing the time and the system will default to the start of the day, so I created a "Before Save" rule:
$(if Find("12:00:00 AM", ToString(FIELDNAME)) != "-1"
then Replace(ToString(FIELDNAME), "12:00:00 AM", "05:30:00 PM")
So, if the user left the default 12:00 AM, the rule will change it to 17:30, the end of the day.
Not sure if it will work for you...
Could you also set the current time, it seems more useful. Seems like the function might be Now()
Not really, if you are going to select "Now()" you could do it with an Initialization rule, here I was trying to set the end of the selected day if 12:00AM was selected, but yes, maybe the EndofDay(function would have worked instead of hardcoding the time...)