Start keynote slide timer on first slide

Add a build or animation that actually does nothing (say text bullet but with only a spacebar character) on your first slide. Click when you start: your timer will kick in but you will still be on the same opening slide.

I'm looking to start the keynote presenter view timer as soon as I click play,

This is how Keynote plays by default.

There is a clock and a timer for presenter display, check you are displaying the timer by going to:

Play > Rehearse Slideshow

move the mouse to the top right of the screen to show the presenter tool bar

click the presenter display options (circled red)

then click in the selection box next to timer

lastly, select Elapsed or Remaining:

My pleasure! 🙂