The Month Between New Moon and New Moon

The length of the month from new moon to new moon is almost exactly halfway in between at 29.53 days. But because the Babylonians had to have either 29 or 30 days per month exactly, errors could accumulate so that some months may have had 28 or 31 days. These irregular months were considered to be bad omens. Fortunately with careful observation, the Babylonian astronomers could anticipate them by tracking when they saw a full moon. If it occurred on the 14th day of the month, all was well. But if it came on the 13th of the month,. and if it came  on the 15th or later, it would be long.