Now, MarGavriel informs me that the 18 minute calculation comes from the Trumat haDeshen, who says: חלק שלישית שעה פחות חלק ל' מן השעה - that is, 1/3 of an hour - 1/30 of an hour, or 20 minutes less 2 minutes in modern idiom.
18 minutes caught on, for whatever reason. Some say it's slightly more, some say it's slightly less - the internets suggest 22.5 and 24 minutes as other possibilities - so the choice of 18 minutes for $shortish_time_period is slightly arbitrary, but the choice of $shortish_time_period for matzah-making isn't.
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Date: 2010-04-23 07:15 pm (UTC)חלק שלישית שעה פחות חלק ל' מן השעה
- that is, 1/3 of an hour - 1/30 of an hour, or 20 minutes less 2 minutes in modern idiom.
18 minutes caught on, for whatever reason. Some say it's slightly more, some say it's slightly less - the internets suggest 22.5 and 24 minutes as other possibilities - so the choice of 18 minutes for $shortish_time_period is slightly arbitrary, but the choice of $shortish_time_period for matzah-making isn't.