Basic date time arithmetics operations

Hello,

For basic date time arithmetics operations, AFAK, there're 
actually the function difftime() and the (dt + num) operations.

I'm wondering if other basic operations exist, like
add(dt, num, unit) where unit would be "y", "q", "m", etc. 

Also for the function seq.dates (or seq.POSIXt), the 
case for by="months" would be more useful if it takes
the situation of the end of month into account. 
For example,

seq(ISOdate(2002,1,31), by="months", length=3)

"2002-01-31 13:00:00 CET" 
"2002-03-03 13:00:00 CET" 
"2002-03-31 13:00:00 CET"

rather than

"2002-01-31 13:00:00 CET" 
"2002-02-28 13:00:00 CET" 
"2002-03-31 13:00:00 CET"

Thanks
--
Fan

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