I don't see this behaviour in my version (7.2.2). I created two users with different cultures (US and Japanese). When they manipulate a cubeview in Data Explorer, pointing to a NonFinancial account in V#YTD, with a column with Cell Type set to Date or Date Time, whichever value they set will get saved in the database as it is - so both users see exactly the same value, not going through any UTC-related mangling.
As far as I can see, OneStream is simply taking the datetime object and saving the value of its .Ticks method. If you want to manipulate it for some reason (e.g. intercepting it into some event handler), you can reconstruct the object:
Dim myDate as new Date(yourDataCellObject.CellAmount)
