AIBU to ask which one of these is correct? One for the pendants maybe...?

[Pusheenicorn]

Which of these is correct???

"... which will be payable monthly, on the second to last working day of the month."

OR

"...which will be payable monthly, on the second last working day of the month."

Me and my boss have had such a row over it I thought she was going to sack me lighthearted.

Oh crikey!
I want to draw a diagram! But i cant.
Second to last means I'm second. Behind the last.
How can you have two last days?
-rushing for the gin bottle unsure how to explain it even to myself-

Use the dictionary?