how to get XCode to add build date & time to Info.plist file

Finally... after a couple years of watching and a month of participating, I have a chance to ask you guys a question of my own.

My boss doesn't trust me (or any process) to increment a build number, he also wants to have a build date & time baked into the app. I'd like to put this into the usual Info.plist file.

I found this related question:

Build information in iOS Application (date/time app was built)

and based on the answers there, I went into the Scheme Editor and added the script below to the "Post-Action" section of the Build phase:

infoplist="$BUILT_PRODUCTS_DIR/$INFOPLIST_PATH"
builddate=`date`
if [[ -n "$builddate" ]]; then
    defaults write "${infoplist%.plist}" BuildDate "${builddate}"
fi

In XCode, my Scheme Editor window looks like this:

_{(source: myke at maniac.deathstar.org)}

Unfortunately, BuildDate never gets written into Info.plist.

Changing "${builddate}" to "$builddate" doesn't work either. I added this line to the script:

echo "build date is $builddate" > /tmp/result.txt

and the date appeared perfectly fine in the written out file. Writing strings into the Info.plist file from the above script works perfectly fine, annoyingly enough.

So, summed up, how to get the date to be added to the Info.plist file?

The code in Michael's answer is incorrect or no longer up to date. The version below fixes an error in the set syntax and also supports build paths with spaces in them.

infoplist="$BUILT_PRODUCTS_DIR/$INFOPLIST_PATH"
builddate=`date`
if [[ -n "$builddate" ]]; then
    # if BuildDateString doesn't exist, add it
    /usr/libexec/PlistBuddy -c "Add :BuildDateString string $builddate" "${infoplist}"
    # and if BuildDateString already existed, update it
    /usr/libexec/PlistBuddy -c "Set :BuildDateString $builddate" "${infoplist}"
fi

Note: This change was submitted as an edit but got rejected and I don't yet have enough reputation to post a comment on his answer...