Time difference in months? (difftime, units)

Is there a way to calculate the number of months between dates?

StartDate <- strptime("01 March 1950", "%d %B %Y")
EventDates <- strptime(c("01 April 1955", "01 July 1980"), "%d %B %Y")
difftime(EventDates, StartDate)

So, there are 61 months between 01 March 1950 and 01 April 1955. There are
364 months between 01 March 1950 and 01 July 1980. What I want is for there
to be a "months" argument to units in difftime. Anybody have a bright idea?
Ah, but months are of variable length.
Is there a better way to approach this than difftime?
Pretty easy: convert to POSIXlt, then use something like

12*x$year + x$month

and subtract those.  You could set up a class for "months" and have 
as.months and a `-' method ....
Thanks in advance, Andy

version
        _
platform i386-pc-mingw32
arch     i386
os       mingw32
system   i386, mingw32
status
major    2
minor    0.0
year     2004
month    10
day      04
language R

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Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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Time difference in months? (difftime, units)

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