ESTA's are currently not required for entry into the US via road, only by air.
Upon entering via road you will be required to fill in an I-94W form, which is the equivalent of the ESTA for land entry (I-94W also used to be used for air, but were removed shortly after the ESTA process was introduced).
If you leave by road you will return the I-94W form when exiting the country. If you leave by air, you should give the I-94W card to the airline at check-in and/or at the gate (different airlines have different procedures). Even if you do leave by air, an ESTA is still not required.
If you do choose to apply for an ESTA (there's no disadvantage other than the $14 charge, but really no need to do so) you should find that the sections regarding travel dates are optional and can be left blank.