selective breeding isn't genetic engineering.
the smell could be because of a couple of things. I notice it says:
"Floral scents begin as oils that are produced by the petals in most plants. Because these oils evaporate easily in warm weather, scientists call them volatile compounds"
so it could be that it's cooler in the evenings so the oils evaporate slower and so there is more to smell.
it also says:
"but many moth-pollinated flowers produce scents only at night, and not earlier in the evening."
Obviously different flowers produce stronger scents at different times. Some flowers only open at night, others only open during the evening, etc. Not all flowers smell stronger in the late evening as not all flowers are open in the late evening :P
It could be that the flowers you have in mind start producing more scent in the late evening to get ready for nocturnal pollinators such as bats, most sphinx moths, etc, and you think it's just in the evening because you're asleep during the night :P
or it could just be that that particular flower's pollinators are crepuscular (most active during dawn/dusk)
or it could be that the particular flower's diurnal and nocturnal pollinators overlap in the evening so it smells stronger because there are more pollinators to attract.
http://www.sciencedaily.com/releases/2000/07/000703090751.htm