29 weeks = 29 weeks * 7 days/week = 203 days

When we do "normal" subtractions, we sometimes need to borrow from a higher rank digit.

For example: 63 - 18

we start with the units (3 - 8) and find that impossible: you cannot take 8 from 3.

You borrow "one" from the 6, leaving behind a 5, and that "one" you borrowed from the tens, now becomes 10 units. Added to the 3 units you already had, you now have a "13" in the unit column, and you can do 13-8 = 5

The tens are now 5-1 = 4

final answer 63 - 18 = 45

We will do the same thing (borrowing) except that each month will have a different number of days.

I am doing this example on June 14, 2018

Our operation:

"June 14" - 29 weeks

is the same as

"June 14" - 203 days

I borrow from the months to bring me back in May (which has 31 days)

June 14 is equivalent to May (14+31) = May 45

May 45 is equivalent to April (45+30) = April 75

and I continue, in this way, until the "date" is greater than 203.

April 75 ≡ March 106 ≡ February 134 ≡ January 165 ≡ December196 (2017) ≡ November 226

(the symbol ≡ simply means "is equivalent to")

Our original subtraction is now:

"November 226" - 203 = November 23 (2017)

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In astronomy, where we observe events over very long periods, we use something called the Julian Date. It is simply a count of days from a pre-determined date

At noon (UT) on June 14 2018, it is the beginning of day number 2,458,284

Take away 203 days, it brings you back to day number 2,458,081

which began at noon (UT) on November 23, 2017

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June 14, 2018 is a Thursday. Since we subtracted an exact number of weeks (therefore, a number of days divisible by 7), then we must end up on a Thursday. Sure enough, November 23, 2017, was a Thursday.