[R] roll weekly data to monthly level

[liujb]

Dear R users:

I have a data set that looks something like this:
ID    time           y
1    2/01/2008    4
1    2/09/2008    12
1    9/01/2008    8
2    1/06/2008    3
2    3/01/2008    4
2    3/09/2008    9
2    6/03/2008    4
3    1/02/2008    3
3    1/10/2008    8
3    2/02/2008    7
3    2/10/2008    3

I'd like to sum up the weekly data to the monthly level, so that it looks
something like this:

ID    time       y
1    2/2008    16
1    9/2008    8
2    1/2008    3
2    3/2008    13
2    6/2008    4
3    1/2008    11
3    2/2008    10


What is the best way to do it?

Time must be character. How do I truncate a character so that I can remove
the date and only keep the month and year?

Thank you very much in advance.
Julia
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