Try the derivation given by ArndW and another way I suggest to do it as a substring, like date and time. U can change the date by using stringtodate() and keep the time as string to tranform it for am/pm using stage variables.
I have resolved it by checking with '/' of each and every combination of timestamp.I have defined 6 stage variables to convert.I cann't post variable derivation as it is very long.
I hope we can write a routine for that too.
Thanks