String to date/Int in workflow editor

Is it possible to convert a string field into a integer within a workflow?  We have a couple of time fields (start and end) that take the time as a string e.g "10:30".  I would like to does some comparisons on these.  I can take a substring to grab the hour or minutes but it's still a string and therefore not useful.
The reason we use string values at all is that date values bury the time too deeply - we need to be able to see them at a glance.  Any suggestions?

Thanks