(not a lawyer) I think in the US (oops, probably should have said California) generally accrued vacation has to be paid off (as it is accounted for at the time

(not a lawyer) I think in the US (oops, probably should have said California) generally accrued vacation has to be paid off (as it is accounted for at the time it is earned, but there may also be a cap which causes some to disappear). But sick days are not generally considered a benefit and avoid this. I imagine "flex time" is more complicated.