Set up recurring transactions for last day of the month

I want my payslips and purchase invoices to recur on the last day of each month.

You can follow the guide here:
Recurring Payslip & Recurring Purchase Invoice

As the Guide explains, that cannot be done.

Dates relative to recognised reference dates are useful in several places in Manager. What you are wanting to specify is one day before the start of each month. A general solution to which is described here Datepicker Defaults - Feature Request - #2 by Patch