AppleScript: How to extract section from string (based on defined characters)?
You can do it with pure AppleScript, even though the shell script method presented above is very nice:

set theText to "I ate an apple at 11:54 pm without the skin."
set the_colon_location to offset of ":" in theText
-- now we know where the colon is.
-- The Time is going to be on either side of it.
set the_starting_point to the_colon_location - 2
set the_ending_point to the_colon_location + 5
set the_time_string to characters the_starting_point thru the_ending_point of theText as string
-- in case the hour is not two digits, the first character will be a space
if character 1 of the_time_string is " " then
    set the_time_string to characters 2 thru -1 of the_time_string as string
end if
return the_time_string