Type (copy) into Terminal:
sudo /Applications/TextEdit.app/Contents/MacOS/TextEdit /System/Library/CoreServices/SystemUIServer.app/Contents/Resources/English.lproj/Localizable.strings
After you type your password, TextEdit will come up and it will open a text file (Localizable.strings) with root editing powers. Search for these lines:
/* Format screencapture file names */
"%@ %@ at %@" = "%1$@ %2$@ at %3$@";
Between the quotes on the right side of the equals sign is the date and time addition. Just edit it, something like this:
/* Format screencapture file names */