I'm in the midst of designing a graphic for my parents' 50th anniversary celebration, and for obscure reasons I want to include the proper Latin equivalent of "since 1967", in the same way that a business might include a founding date on a sign or logo, such as "est. 1950" or "ever since 1995". My research on the web has thus far been inconclusive, indicating that either "ex MCMLXVII" or "ab MCMLXVII" would be most appropriate, but I am unable to determine which is more appropriate (or if there is some other way of putting it entirely that would be preferred).
What's the best way to write a founding date for an ongoing entity (or the date of the beginning of a marriage, if that differs from the general case)?