I've made a script for AutoHotkey which means you just have to press a button (or combo) to start the timer and press play at the exact same time.

Those who have used AHK before should know how to manipulate it to fit them, otherwise:

Once you've got AHK, go on your desktop, right click, New, and AutoHotkey Script (if it isn't there you've done something horribly wrong). Right click and Edit, and then delete the text already there, and then paste this:

^m:: SendMode Input CoordMode, Mouse, Screen Click 265, 124 Send {Space} Click 1781, 1022

The first line is the shortcut entered, which is currently Ctrl + M. To change this, look at https://autohotkey.com/docs/Hotkeys.htm and change the "^m" part to your desired hotkey.

The first click command clicks on LiveSplit through a coordinate. However, if you've placed LiveSplit somewhere other than the opening-up place, you need to change this. Create another script, and paste this into it:

^p:: CoordMode, Mouse, Screen MouseGetPos, xpos, ypos Msgbox, The cursor is at X%xpos% Y%ypos%.

This will tell you the coordinate of your mouse cursor when you press Ctrl + P. Hover the mouse over LiveSplit, press Ctrl + P, and replace the first Click value with the two numbers.

The Send command will tell LiveSplit to start, and if the starting key for LiveSplit isn't Space, you'll need to change this by choosing an output from https://autohotkey.com/docs/commands/Send.htm .

The second Click command presses Play in fullscreen. If you don't play in fullscreen, you'll need to change this value by using the method for the first Click.

Remember to click on LiveSplit before you start the script.