Interesting... don't recall this one coming up before. This could be a new conditional option, giving a date/time range for the field to be activated. We'll consider this for future development certainly. If you'd want to discuss it as a paid customization project, let us know.
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Another older post in this forum suggested field become invisible x days prior to the event. He wanted to offer a free t-shirt to those that enrolled early.
Perhaps an options for dates or x amount of days would serve a lot of purposes.