Absence start date validation requires another reference date value: "1st day on month"

Summary

We need to apply a validation on absence start dates thus no one will be able to enter an absence for the previous month,

Content

We need to apply a validation on absence start dates thus no one will be able to enter an absence for the previous month, but the only refernce date option when writing a rule for Absence start date validation is "System Date", how can we add other refernce dates here?