Default Values - Can they default only when first used?

I am noticing that if I have fields (time fields) that I want to default to the current time when they are used, it is actually defaulting to when the record was created.

For example, if I have a start time and end time field in an object, but I don't display that field when I first create the record, it gets prefilled with the current default date. I actually will not use that field until the user accesses another form (think of the setting up a survey example). If I show the field for the first time on a form, it is already defaulted based on when the record was created.

I have options, like keeping the time field in a separate object and creating a new record each time, but I would prefer to keep the field in the current object and edit that object and have the time default with the field is first used on a form.

Make sense?

Thanks for your help,