its not the 11/13 or whatever that you will lose days. if the two numbers add up to 24, you will not lose or gain hours/days etc over a 12/12... Now, u do lose a couple of weeks running an 18 hour day. 6 hours on and 12 hours off. here is how. u need a digital timer to do this.
if they take 56 days to mature, that;s 56 times they gotta get 12 hrs of darkness.
56 times of 12 hrs, equals 672 hours time needed for darkness.
now, if u run a standard 12 hour light time, then that means its double the 672, 12 more hours each period. 672x2 = 1344 hours or, 56x 24hour days.
ok, so if u only run your lights 6 hours a day. the you're cutting your 24 hour "day", into a 18 hour "day". so it's only 6 hours per period, rather than 12 hours of "light time"..
so, that is 627/2 = 336. 336 hours are the time the lights are on.
so, 336 light + 672 darkness = 1,008 hours.
now switch back to a 24hr day, and divide 24 into 1,008
it's 42 days. So yeah, u can make a 8 week strian flwer in 6 weeks.
the drawbacks are that you will loose 15%-30% yield, depending on strain and all the million other factors and variables that go into growing.. and the lights come on and off at different times each calendar day, and its confusing and annoying.. the 18hr day is a specific, niche, light time that is used for quick turn over when yur short on time. its not for everyone..