<p>I have a sqlite3 table that I'm trying to map to an object in objective-C. One attribute of the table is 'completed_at' which is stored as a DATETIME.</p>

<p>I want to create a property on my objective-C class (which inherits from NSObject) that will map well to the 'completed_at' attribute.</p>

<p>Objective-C has an NSDate type but I'm not sure if that will map directly?</p>

I have zero experience with Objective-C, but I found Apple's
NSDate Class Reference
with a google search. With the information provided on the linked page you should be able to figure out how to manipulate 32-bit epoch times in Objective-C, and this would work well in SQLite. I would probably create the completed_at column as type INTEGER for 32-bit times.
SQLite really prefers Julian dates, which are floats. I haven't found any documentation explaining how one might coerce the NSDate class into working with Julians.
timeIntervalSince1970
looks very interesting.