I wonder how I can make use of an already existing date field when creating a ts in R. Sometimes you simply have a date before you have a ts object, e.g. 

x <- as.Date("2008-01-01") + c(30,60,90,120,150)
# add some data to it    
df = data.frame(datefield=x,test=1:length(x))

Now, is there a way to use the datefield of the df to as an index when creating a ts object? Because:

   ts(df$test,start=c(2008,1,2),frequency=12)

(obviuously) completely ignores the date information I already have. Making use of ts methods like acf is the reason why I´d like to make it a ts object. I typcically use monthly an quarterly time series...

r time-series
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