Coyotes can be killed anytime of day or night... you just need to get within a distance of the coyote that he feels safe traveling at that moment in time. They're more comfortable in darkness but also confident in daylight/dusk situations where light levels are low. They'll come hauling ass to the gun in the broad daylight- if your setup is correct.
Check out this link for some cool pictures of a pair of daylight coyotes I called recently-
http://midwestpredator.groupee.net/eve/forums/a/tpc/f/6041018811/m/3311023513