But with an older MonetDB, "second"(date1 - date2) would be "second" of an integer, which makes no sense.

Of course also the cast would not solve everything, because it wouldn't solve the underlying fact that, for example, a date difference returns days (int) in one case and a second interval in the other. So it's not only the formal type that has changed, but also the scale.

Still one could argue that the cast is transparent. You just change the formal type and you deal with the scale.

In the meantime I also got some more details about why keeping the cast would have been problematic, which I understand.

It's unfortunate that this change implies a difficult redeployment strategy of the existing applications, but maybe it was indeed hard to avoid this time.