By default Impact's document merge merge engine uses standard formatting for date and time fields in ACT! These field will be formatted as such "06/24/2014". You can change the formatting rules globally by modifying a configuration file.
To see more formatting options please read Printing Format
1) Close ACT!
2) Go to START - Durkin Impact v4 -> Durkin.Common.DocEngine
3) Edit the file "Generic.v4.Config.xml" using NotePad.exe
4) Find this section and change the value from {0:d} to {0:D}
5) Save the changes then restart ACT!