[R] data prep question

[Matthew Strother]

I have a data set with several thousand observations across time, grouped by subject (example format below)

ID		TIME	OBS
001		2200	23		
001		2400	11		
001		3200	10	
001		4500	22
003		3900 	45	
003		5605   	32
005		1800	56
005		1900	34
005		2300	23
...

I would like to identify the first time for each subject, and then subtract this value from each subsequent time.  However, the number of observations per subject varies widely (from 1 to 20), and the intervals between times varies widely.   Is there a package that can help do this, or a loop that can be set up to evaluate ID, then calculate the values?  The outcome I would like is presented below.
ID		TIME	OBS
001		0		23		
001		200		11		
001		1000	10	
001		2300	22
003		0	 	45	
003		1705   	32
005		0		56
005		100		34
005		500		23
...

Any help appreciated.