Meaning of BMNT
BMNT stands for Begin Morning Nautical Twilight. It is the morning crossing of the Sun’s geometric center through 12 degrees below the horizon. In ordinary mid-latitude conditions, it falls after astronomical dawn but before civil dawn and sunrise.
Why a fixed offset is wrong
Do not subtract a constant number of minutes from sunrise to estimate BMNT. Latitude and season change how quickly the Sun climbs through the twilight angles. The calculator searches the altitude crossing for the chosen city and date.
A missing crossing can be normal
In high-latitude summer the Sun may never descend below −12 degrees. There can then be no BMNT event on a particular date. A dash does not mean midnight or an unavailable internet feed. Compare adjacent dates and the solar-altitude curve.