noa2 hardcopy....what does valid dates mean specifically?

[EegyptA 41]

just got the noa2 letter approval hardcopy in the mail today....on the right hand side reads....

notice type: approval notice

valid from: 4/22/2013 - 8/21/2013

consulate: Cairo

does this mean his interview date will be by august 21st 2013,,,,,,or does that just mean they have to give a date wether it be after august or whenever by 8/21/2013 before the petition expires?

[meadow 2]

My NOA2 had that as well. If I understand correctly, the noa2 is good for 4 months, which shouldn't be a problem if you continue with the process and schedule the interview within that time. I think that after four months, it expires and you'd have to apply to extend it to be able to continue.