From the manual:
"The "yearmon" class is used to represent monthly data. Internally it holds the data as year plus 0 for January, 1/12 for February, 2/12 for March and so on in order that its internal representa- tion is the same as ts class with frequency = 12."
> require(zoo)
> as.yearmon("2007-03-01")
[1] "Mar 2007"
> as.numeric(as.yearmon("2007-03-01"))
[1] 2007.167
Is this what you really want, a yearmon which is a fraction of a year? If this works for you see