[R] converting a data set to a format for time series analysis

[Ricardo Pietrobon]

I currently have a data set describing human subjects enrolled into an
international clinical trial, the name of the hospital enrolling this
human subject, the date when the subject was enrolled, and a vector
with variables representing characteristics of the site (e.g., number
of beds in a hospital).  my data sets looks like this:

subject	hospital	date_enrollment	hospital_beds
1	hospitalA	1/3/2002	300
2	hospitalA	1/6/2002	300
3	hospitalB	2/4/2002	150
4	hospitalC	3/2/2002	200

to perform a time series analysis I am now trying to get to a format
that would give me the following variables:

month	year	site	number_enrolled_subjects	hospital_beds

the data would be displayed on one-month intervals, and number of
subjects clustered around sites.

any help would be greatly appreciate

thanks


Ricardo