I understand the problem stoux, there is also a problem determining the time a player has been on a server, but basically, yes, about that, with a few restrictions: must have been a member for that time - that means, a forum account.

A different parameter could be whether a person was a VIP or not in the past, it is most likely that members from a year and a half ago that have VIP can be considered Veterans, they have enough devotion to the server. The VIP condition could serve as a time saving factor, like, if one was once VIP and joined a year and a half ago, he could be considered a Veteran, while a member who joined at the same time, who never had VIP is less likely to be considered.
Then again, hard indeed.

Maybe Letting those who joined back two years ago can be a fitting solution, regardless of their experience, if they had a plot could be a factor, but it is not much of a thing you can automatically do.

Another factor may be whether a person was banned or not. And if a guy from 3 years ago asks why is he a veteran.... can always demote...

it's a bit exploitable, in a way, but hey, don't they, who tried our server a long time ago and gave another look now worth something?
Who are we to say no, once a member always a member.