[R] Need guidance on summarizing time data

[Neotropical bat risk assessments and acoustic tools]

Hi all,

I need to summarize temporal activity. However date\times in R seem to 
be not easily handled.
Seems I may need to convert date\time values to a recognizable format?

My "raw data" is tab (text) includes a location ID, date and time(24 hr 
format).
Format is like this:
Location    Date    Time
156    2/25/2008    18:31
156    2/25/2008    18:31
156    2/25/2008    18:32
156    2/25/2008    18:35
156    2/25/2008    18:38
156    2/25/2008    18:41
156    2/25/2008    18:42
156    2/25/2008    18:43
156    2/25/2008    18:43
156    2/25/2008    18:55
156    2/25/2008    18:56
156    2/25/2008    18:56
156    2/26/2008    18:35
156    2/26/2008    18:35
156    2/26/2008    18:35
156    2/26/2008    18:35
196    7/16/2006    4:47
250    4/9/2004    18:41
250    4/9/2004    18:44
253    3/5/2004    18:30
1268    2/11/2001    18:39
1268    2/11/2001    18:39
1344    4/17/2003    19:06
1409    2/28/2004    5:51
...etc. for 10,390 rows of data.

I am aiming for a summary by times for all of the data such that I have 
total number of "events" or count for each time period. So something like

18:31 41
18:32 38

and so on.

So a "simple" count of the time occurrences.

I tried to do a summary running frequencies
descriptive.table(vars = d(Time) ,
+ strata = d(Date),data= Active,
+ func.names =c("Valid N","Minimum","Maximum"))


Warning message:
In descriptive.table(vars = d(Time), strata = d(Date), data = Active,  :
   Non-numeric variables dropped from descriptive table
 > Active[,3]<-as.POSIXct(Active[,3], format='%m-%d-%y %H:%M:%S')
 > Active[,2]<-as.Date(Active[,2], format= '%m/%d/%y')
 > frequencies(Active[c("Time")] , r.digits = 1)
Error in names(x) <- value :
   'names' attribute [4] must be the same length as the vector [3]

Suggestions welcomed.

Cheers all

-- 
Bruce W. Miller, PhD.
Neotropical bat risk and acoustic assessments
Conservation Fellow - Wildlife Conservation Society
Research Associate, American Museum of Natural History

If we lose the bats, we may lose much of the tropical vegetation and the lungs of the planet

Using acoustic sampling to identify and map species distributions
and pioneering acoustic tools for ecology and conservation of bats for >25 years.

Key projects include providing free interactive identification keys and call fact sheets for the vocal signatures of New World Bats