Conditional Time Field on Notes Form?

I'm creating a form that will hold several fields, but I'm not sure how to handle one of the requirements.  They're asking for two from and to time fields, essentially an event start time and event end time.  Simple enough, but there will be some cases in which the end time will not be provided, because the issue is ongoing.  So normally, the field(s) will read like this:

Event:  10:15  to  13:35
Which will be simple, as my plan was to create a table row with "Event:" then the start time, then a text field with "to", then the end time.  But they want to optionally select a checkbox to indicate that the event is still ongoing, so when it is selected, this is what the field(s) will display:

Event:  10:15  to  (Ongoing)

Obviously I can't store a text string in a Date/Time field.  I'm thinking that I would need to say that if they check the "Ongoing" checkbox, instead of an end time field, I would substitute a text field with the word "(Ongoing)" as its value and conditionally hide the end time field?

Am I on the right track, or is there an easier way to do this?  Any help would be appreciated!