Thanks for the reply.
A bit confused, since this is not documented anywhere on the GOV UK site, or during the application process. We weren't even able to find it in the guidance given to UK immigration employees who are the decision makers on these applications. I understand the UK fiance's visa, similar to the U.S. fiance's visa that we applied for a few years ago, has a 3 month advance time limit, but this was a family of a settled person application.
We did not have any issues inputting a future date for our intended travel, since my contract begin date is January 1 with my UK employer. We've seen this noted by others, but no one has been able to provide any actual government link or document that states this advance time limit for family of a settled person visa.
In addition, we've already found documentation that allows my wife to extend her passport vignette if she is unable to travel within the time limit on the visa.
Thanks