Minimum time entry in Timesheet

In our company, we record all times worked on projects in 5 minute steps, with a minimum of 15 minutes per entry. How would I go about customizing something like this for the timesheet?

I’m not expecting a free step-by-step explanation, I’m basically trying to figure out if this is something that could be achieved with the kind of customization available in the administration section of ERPNext (using client script / server script) or if changes to the Framework itself would be necessary (such as described here)