The following is posted on behalf of @jerome:
I assume you are talking about US releases.
You can check out the Online 78 Discographical Project, which, indeed, gives recording dates, but you can count on the fact that records were released within a month after ther recording session, in most of the cases. There are exceptions though, but you can easily find out by confronting the recording date and the dates of the catalog number. Some releases have been postponed or simply repressed.
You can also check http://www.45worlds.com/78rpm/ where many release dates are provided (based on the billboards found on Google Books).