Just replaced mine. The old one was three years old and on the last trip of 2015, it chirped like crazy. As someone else did, I disconnected it, took it outside and connected it to my truck battery, placed it on the ground and within minutes it started to chirp. There was NO replace date on the unit, but per the manufacturer MTI, there is a production date and the unit should last five years. Mine didn't; they didn't seem to care.
Due to the size, I ordered another one from Amazon. It had a large red stamped date of AUG XX 2015 on the back. So we'll see if it lasts till 2020.