I agree with you, S-Cat. A definitive date after which the monogamous period concludes sounds like a fine idea.
Honestly, I think couples who are good with and for each other should be able to make this transition -- but only if the more resistant partner comes to understand certain facts about polyamory which the general population doesn't "get". For example, they need to understand that love is not made less by sharing with others. One can only really--or fully--know this in experience. Or maybe it just needs to be confirmed in one's own experience before it can be believed. But it is a fact, nevertheless. It is a fact of many people's experience.