I am successfully running a cron job every hour on Google Appengine. However I would like it to start when I launch the app. Now it does the first cron job 1 hour after the start.
I am using Python.
There is no "launch" the app in production as such.  You deploy the app for the first time and crontab is now present and crontab scheduling is started. So I assume you really mean you would like to run the cron job every time you deploy a new version of your application in addition to the cron schedule.
The cron handler is callable by you, so why not just wrap appcfg in a script that calls the cron handler after you do the deploy. Use wget/curl etc.....
Cron job on Appengine - first time on start?