AFAIK there isn’t a way to get the API you want, i.e. property x : Epoch = 1589831432. It would have to be like what @wrq suggested. property x : Epoch = Epoch.new 1589831432.
Could possibly be a new compiler feature? That is given x : Epoch = 1234, have that be expanded to x : Epoch = Epoch.new 1234. But thats just an idea…