[R] Aggregating on date at Monthly Level

[Shivi82]

HI All, 
I have situation where i am aggregating weight on monthly and quarterly
level.
I need to summarize weight on variable ship date i.e. shipping date . As
this date is in a character format so used the conversion as:

Shipdate<-as.Date("retail$ship.date", format="%m-%d-%Y"). But when i see the
output of Shipdate the result is NA. 
typeof(Shipdate) gives me "double". I am not sure what this is var type
whereas class (Shipdate) is Date so i am a bit puzzled. 

Then i have created a new var which sums weight and gives me the desired
result- 
aggr<-aggregate(retail$weight,list(retail$ship.date),mean)

Now if i want to group the result on a Month level i used the below syntax
and gives me error:
cut.Date(aggr,breaks = "Month",labels = TRUE). "x must be numeric"- in the
above syntax i have created it as a list.

Is there an easier way to achieve what i am looking for. Kindly suggest. 
Thanks, Shivi



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