[Info-ingres] Doing arithmetic with ANSI dates

[Roy Hann]

I'm well used to doing arithmetic with the native Ingres DATE
(INGRESDATE) type, e.g.:

   SELECT birthdate + '3 months' FROM...

If birthdate is an ANSI date (ANSIDATE) the above still works
perfectly well.

But I doubt that is idiomatic ISO/ANSI SQL. I've Googled a bit and found
all kinds of syntax being used. I suspect ...+'3 months' or ...-'49
days' might not work reliably outside of Ingres.

So is there anything that does work reliably? Google is not being super
helpful on this...  Maybe I haven't guessed the best search term
though.

Roy