Creating a new Grails tag to convert military time to standard time

I have a situation where I need to store time as military time (i.e. 0900 is 9:00 AM), but I want to display it as standard time. So, I created a new tag library in Grails to perform this for me. I then simply use the new tag in my gsp page, and all is well. Here's the code you would put in your taglib directory:

class MyTagLib {
   def standardTime = {attrs, body ->
       out << showStdTime(body())
   }

   def showStdTime(militaryTime) {
       def hours
       def minutes
       def ampm = "AM"
       if (militaryTime.trim().size() != 4) {
           return militaryTime
       }
       try {
           hours = new Integer(militaryTime[0..1])
           minutes = militaryTime[2..3]

           if (hours > 12) {
               ampm = "PM"
               hours -= 12
           }

           if (hours == 12) {
               ampm = "PM"
           }

           if (hours == 0) {
               hours = 12
           }
       } catch (Exception e) {
           return "Format Exception: " + e.toString()
       }

       return hours + ":" + minutes + " " + ampm
   }
}

And here's the tag as it looks in my gsp page (substitute the lt for <>. I couldn't get the formatting working right):

<g:standardtime>${myClass.closingTime}</g:standardTime>