The date is arbitrary and refers to the date when you both decided to commit to a life together. You may or may not have proof of that date and may or may not have witnesses but try to find some way of affirming the date."When did you and your fiancée committ to a shared life together to the exclusion of all others"
There may have been other things happening on or near that date that can be used as evidence. Photos at a restaurant or family gathering. I used a Western Union transaction screenshot for support money I sent the day after we committed to our shared life together as evidence.