Set up discount codes that "expire" on a certain date (by going back in to the trip builder and deleting them at that date).
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Set up dedicated early bird pricing packages that expire on a certain date (you'd need to set up each pricing package 2x, one with standard pricing and the other with early bird pricing for each case, with the early bird packages set to expire on the right date).
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Go back to the trip builder and change the pricing on a certain date.