If you're sure this is something that you want to do, I think the best way to try it is to use the
tmutil (Time Machine utility).
Begin a backup if one is not already running.
--auto Run the backup in a mode similar to system-scheduled backups.
--block Wait (block) until the backup is finished before exiting.
--rotation Allow automatic destination rotation during the backup.
--destination Perform the backup to the destination corresponding to the specified ID.
The --auto option provides a supported mechanism with which to trigger "automatic-like" backups, similar to automatic backups that are scheduled by the system. While this is not identical to true system-scheduled backups, it provides custom schedulers the ability to achieve some (but not all) behavior normally exhibited when operating in automatic mode.
Combine that with
launchd and you can have
tmutil run every 15 minutes, take this:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
and save it as
~/Library/LaunchAgents/com.tjluoma.time-machine-intervals.plist and then load it into
launchd with this command in Terminal:
launchctl load ~/Library/LaunchAgents/com.tjluoma.time-machine-intervals.plist
Here's how it looks in LaunchControl which is my favorite app for working with