The seventeenth day (7:11) comes seven days after the warning received a few verses earlier (7:4 and 7:10), which means that the warning itself came on the tenth day of the second month, which is the fortieth day of the year, yielding an intriguing symmetrical structure :
forty days from Rosh ha Shanah until the warning,
then a full week between the warning and the beginning of Noah's Flood,
followed by forty days of heavy rains and massive inundations,
mounting up to a total of eighty-seven days, about the length of a season, spanning roughly from the autumn equinox to the winter solstice.
The forty-day periods are ultimately of Egyptian origin (50:3), since they divided their years (which, just like Noah's Flood, started in autumn, with the flooding of the Nile) into twelve months of thirty days each (7:24 and 8:3), the months being further subdivided into three decans, having ten days each.
However, the middle time-period, instead of spanning for a full Egyptian decan, is instead three days shorter, having the same length as a Babylonian week1, with whose lunar calendar the ancient Hebrews were also intimately acquainted, eventually adopting it as their own. Now, these three days reduce what would otherwise amount to a ninety-day period to one of only eighty-seven, a number which can only be written as the triple of twenty-nine, hinting at the astronomical knowledge that the duration of a synodic month lies somewhere between the two values, being perfectly equal to neither.
Between these two different calendars, Egyptian and Baylonian, there is a difference of about ten days, representing the inequality between the solar and lunar years (7:11 and 8:14).
1 Come to think of it, the time span from Passover to Pentecost seems itself based on an attempt at equating five Egyptian decans by seven Babylonian weeks.