add built-in is not supported yet by HermiT 1.3.1 (20 January 2011).

hasBeginning(?int,?inst1), inDateTime(?inst1,?dt1),  dayOfYear(?dt1,?day1), 
hasDurationDescription(?int,?d), days(?d,?value), add(?deadline,?day1,?value), Instant(?inst2), inDateTime(?inst2,?dt2) ->
dayOfYear(?dt2,?deadline) -> hasEnd(?int,?inst2)