library which convert dates

I'm looking for library  which let mi convert dates

for example like this:
00-06-05 00:00
00-08-06 00:00
00-08-16 00:00
00-05-23 00:00
00-01-14 00:00
00-10-28 00:00

and as a result I want to get a 3 levels
  

Hi Gregorio,
I might be wildly wrong, but I think you want to read in dates like this:

mydatestrings<-c("00-06-05 00:00",...)
mydates<-strftime(mydatestrings,format=%y-%m-%d %H:%M)
myresult<-cut(mydates,breaks=3)

to get the year 2000 divided into thirds. Feel free to ignore this if I 
am wildly wrong.

Jim