Is it true that after ovulation it takes 12-24 hours for bbt to rise? and thus, if you ovulate at night or even late afternoon, your morning temp may not show it yet?

Possibly: Temperature charts are less reliable than ovulation predictor kits. These are available over the counter, simply test by holding the stick in your urine stream.

Essentially true: It can take up to two days for the basal body temperature (BBT) to rise following ovulation. The temperature rise is in response to release of the hormone progesterone. This starts at the time of ovulation and the Progesterone level needs to build to the point that it causes the temperature rise. So the best strategy is to plot temperature over several cycles and so try to predict ovulation.