How to convert a string value in the format HH:mm:ss to usable seconds for a graph?

Hey Gang, We are currently running Splunk Enterprise 6.3.1 on RHEL 6.x servers. I have a string value that I have brought in from a long that represents hours, minutes, and seconds in the form `HH:mm:ss`, and the user would like to graph the trend of the duration. The easiest way that I can think of to do this would be to convert this string into a number of seconds so that I could represent a single numeric value to graph. Does anyone have an easy way to do this, or an alternate way to graph `HH:mm:ss`? Thanks, in advance for any help you may be able to provide.