I've seen and heard about many different sick pay schedules because of this. From ones that accrue like vacation and PTO from zero as hours are worked, ro ones where the days are all allocated on the first of the year to be used (I assume pro-rated for mid-year hires) to ones that are a hybrid where some portion of the yearly sick days are allocated at the beginning of the year and the rest are accrued. There are benefits and drawbacks to each, and probably a lot of interesting behavioral nudges in each scheme that affect different types of work in different ways.