[R] Looking for a special date function in R

[Sergey Goriatchev]

Hello, everyone

I wonder if R has something similar to Excel function
EDATE(start_date; months) which returns a serial number of the date
that is the indicated number of months before of after the start date.
Example (the second column EDATE(first_column; -6)):
01.01.1999	01.07.1998
02.02.1999	02.08.1998
06.03.1999	06.09.1998

I am working with a zoo object where the row names are dates and for
particular rows I need to find values that were recorded 6 months
before (or return NA if the date is before the timeseries start).

Maybe someone knows a passable R function for that kind of operation?

Thanks in advance for help!

Best,
Sergey