that's an awfully late time for an outdoor plant to begin flowering in the northern hemisphere...most strains are getting picked by then or soon will be...unless you had it growing indoors under lights until you set it outside a couple weeks ago. are you sure about the date? flowering should have begun no later than mid-august for the sativa's, a few weeks earlier for the indica's. both, except for some sativa's should be finishing by now, if not already brought in.

and no, it won't prolong flowering time. you just won't get as much or the tight buds you would normally get under stronger sunlight.