Don't think there is an official line. I was dragged to court in the UK, there were 3 pensions, 2 UK ones, 1 non UK one, they all had different dates no them for CETV, indeed the non UK pension didn't have a CETV at all. The judge took the values on the dates, what else could he do?
Similarly, I happened to have shifted some of my kids' money temporarily into an account in my name (kids' money the judge ringfenced) in order to gain extra interest. The judge originally took the "assets" on the day of court and took this 2k in my name as a marital asset. I protested that was completely unfair, given I was trying to gain the kids some more interest and that the same money was being ringfenced and taken out of the marital assets pot, then counted inside it. So I went immediately online and moved the money back the the kids' accounts, the judge then took the assets as 2k less. So only on my own experience, the judge takes values of any family assets as to what he has in front of him as evidence on the day, not what WAS available at time of separation.
Having said that, my husband also withdrew 34k of family assets and for a year denied the existence of these assets, as did his lawyer. Finally on court day and under oath, he admitted he didn't just withdraw this money, he admitted he spent the whole lot in order to deny his children and myself from it being a family asset. So the judge ordered that the 34k be deducted from my husband's settlement. So for this asset, it was the value of the asset, had it not been spent already, on the day of the trial, as interest was also added back onto this 34k too. It was a hellish fight to get my husband to acknowledge the existence of this money and boy did it hurt that a lawyer, despite the evidence shown to her, continued to lie and tell me it did not exist.
So on my experience alone, it's the values ON THE DAY or the values of assets removed, had they still existed, ON THE DAY.