days rest or days' rest or something else

[BB Bobcat]

...as in Carlos Zambrano is pitching on three days rest.

The apostrophe seems unnecessary, but I see it a lot.

 

[Dan Rydell]

Echo the apostrophe.

Three days' rest.

Two years' probation.

An often-overlooked rule.

 

[reformedhack]

I'll third the motion. It's a possessive. Thus "the rest of three days" equals "three days' rest." Ditto for "18 years' experience" -- it's "the experience of 18 years."

 

[BB Bobcat]

Seems odd to me, but I'll buy it.

Thanks guys.