In parsha Breishis (1:5) Hashem separated between day and night. There is a time of the day called Bein Hashmashos. This is a time which we are in doubt weather it is day or night. In Shabbos 34b the Gemara sites a dispute between Rabbah and Rav Yosef on the length of bein hashmashos. This is the period when it is unclear if it is halacically day or night. According to Rabbah it is 3/4 of a mil. According to Rav Yosef it is 2/3 of a mil. A mil is 18 minutes. How many minutes is bein hashmashos according to Rabbah and Rav Yosef?
18 x 3/4 = 13.5 minutes according to Rabbah
18 x 2/3 = 12 minutes according to Rav Yosef