Why does strtotime('x') return tomorrows date

I cant understand this - why does the following occur?

echo date("d-m-Y", strtotime($str));

$str = '214454';  // Produces todays date
$str = '333333';  // Produces 1-1-1970
$str = 'a' (or ANY single char)  // Produces tomorrows date
$str = 'aa' (or ANY double char)  // Produces 1-1-1970

OR just returning the strtotime function

echo strtotime($str);

$str = '214454';  // Produces todays date
$str = '333333';  // returns false
$str = 'a' (or ANY single char)  // Produces tomorrows date
$str = 'aa' (or ANY double char)  // returns false

These values came from some testing I was doing, to try and work out how/why certain values were being returned from a specific function.

Its causing my function to fail - because you would assume "a" (or any single char) to be returned as a false incorrect date.