Hi @ldfgrne,
unfortunately I don’t see a way to achieve this with Apilio at the moment.
The missing feature is a date-time variable with the ability to do modifications like adding 15 minutes.
You could store the end time into a string variable and then pass that back to IFTTT into an applet to create a google calendar entry (see here how to send dynamic values to IFTTT: IFTTT action – additional value parameters)
I haven’t tested it myself, so no guarantee it will work, but maybe worth a try 