My recollection of the times guide is that resturants which don't operate on limited hours take seatings until 15 minutes before park closing. In other words, if DHS closes at 8 pm, your latest possible seating time is 7.45 pm. I expect this is intended for the information of guests who wish to walk up without ADR's, assuming there is a table to accommodate them, so no, they don't clear you out for park closing, but the kitchens generally close for new orders at park closing so the 15 minutes is intended to get the guests in and have their order placed before the clock strikes closed. We walked in somewhere at the MK about 10 min before park closing and they seated us no problem. The place was empty - really nice.
If you are going online for ADR's (which you can do a few hours earlier than phone lines open) and you have trouble with the time you want, try calling immediately when the lines open at 7 am EST. The ADR system is somewhat strange and sometimes if one method of ressies doesn't work, try for the other.