俱舍論記 11.57

11.57

「增一臘縛至夜增晝增」者，答。准下論文，剎那百二十為一怛剎那，六十怛剎那為一臘縛，三十臘縛為一牟呼栗多，三十牟呼栗多為一晝夜。於中晝夜若至極長，有十八牟呼栗多；若至極短，有十二牟呼栗多；中間延促有六牟呼栗多。三十臘縛為一牟呼栗多，三六十八成一百八十臘縛，故晝夜增減各一臘縛。從夏至日已去日行此洲，從北向南說夜增時增一臘縛，晝即便減一臘縛也。若從冬至日已去日行此洲，從南向北說晝增時增一臘縛，夜即便減一臘縛也。南北道路有一百八十，日日行一路。以此文證，故知夜極短已去說夜增晝減，晝極短已去說晝增夜減。

Regarding the passage from "It lengthens by one lava" to "the night lengthens and the day lengthens": This is the answer. According to the treatise text below: one hundred and twenty kṣaṇas make one tatkṣaṇa; sixty tatkṣaṇas make one lava; thirty lavas make one muhūrta; thirty muhūrtas make one day-and-night. Among these, when day or night reaches its longest, it has eighteen muhūrtas; when it reaches its shortest, it has twelve muhūrtas; the difference between the longer and shorter is six muhūrtas. Thirty lavas make one muhūrta; three times six is eighteen, making one hundred and eighty lavas; therefore, the lengthening and shortening of day and night is one lava each. From the summer solstice onward, when the sun moves [over] this continent from north to south, when the night is said to lengthen, it lengthens by one lava, and the day then shortens by one lava. If from the winter solstice onward the sun moves [over] this continent from south to north, when the day is said to lengthen, it lengthens by one lava, and the night then shortens by one lava. The north-south paths number one hundred and eighty, and every day it travels one path. Relying on this text as proof, one therefore knows that from when the night is at its shortest onward, the night is said to lengthen and the day to shorten; and from when the day is at its shortest onward, the day is said to lengthen and the night to shorten.