It is my impression that the transit visa will not have an arrival date or departure date, it has just an issue date and a validity (14 days from issue date), and the 48 hours count from actual time of arrival, but I have a hard time confirming it.
The official UAE site says
The validity of the visa is 48 hours from entry into the UAE, and you must leave the UAE within 48 hours from arrival.
My guess is that the arrival date is just there to make sure the visa is not requested too far in advance (since it would no longer be valid by the time you arrive): you cannot enter a date more than 14 days in the future. The few readable images of 48 hours transit visas I found did not have an arrival or departure date at all.
However I have a hard time confirming it with hard evidence. Your best bet is probably to contact them, either Etihad:
+971 (0) 56 677 0326
+971 2 4955555
If you don't get an answer in time (or you get a negative answer), then you should use a 96 hours transit visa, which requires a fee of 50 AED.
Also remember that you need a hotel booking (or other evidence of someone hosting you) for transits longer than 24 hours.