new type proposal

I've been thinking on some new pesudo-types like SERIAL.

1) LAST_MODIFIED is the timestamp of that last time this row was
modified.  Easy enough to do currently with triggers.
2) TIME_CREATED is the timestamp of when this row was first created with
an INSERT.  I'm not sure how to do this because it needs to be read
only.

Both of these could be useful to me and I can implement them by myself
(except #2 which I'll just have to trust myself not to screw up) but
general users might find them useful.

What do you think?