When was he born?

If you use a calculator, then we can find the day he was born on, at precisely 12:00 AM.

To find the total number of days elapsed, we take 31 + 28 + 31 + 13 and we subtract .2(365) which results in 30. This is the amount of days since January 1st, so that makes it the 31st.

To find the year we look at a relation: he is 18 in 2017, 17 in 2016, and so forth. We can modify this equation to y = x - 1, with no regard to the first 2 digits. If we input 0, then we get -1. However, this function is in relation to the year 2000, so 2000 - 1 = 1999. The birthday is January 31st, 1999.

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Calvin Edit

$(.2)$ years = $.2\times 365$ = $73$ days = ${ 13 }_{ apr }+{ 31 }_{ mar }+{ 28 }_{ feb }+{ 1 }_{ jan }$

Since we are going backwards, $1$ day of January means he was born on ${ 31 }^{ st }\quad January$ .

Since he is $18$ years old, he was born in $2017-18=1999$ .

Hence he was born on the day - $\boxed{{ 31 }^{ st }\quad January\quad 1999}$ .