Finkelstein:
why didn't the WTS. include those extra days in their final calculation of 70 x 360 to reach to 1914 ?
(Presumably, you mean 7 x 360). It was already generally understood by people using the Babylonian calendar (from which the Hebrew calendar was derived) that years didn't actually have exactly 360 days, which was a convenient rounded figure. (But also note the slightly different Essene calendation indicated by Bart.) The ancient and modern calendars both incorporate methods of correcting for the additional days to compensate for seasonal 'drift', so that in both systems, spring always begins in March/April. As a result, in both systems, a year is a year is a year.
That said, there is no actual basis for applying the 'day for a year' 'rule' to the 'times' at Daniel 4:16 anyway, and the Watch Tower Society does not apply the same 'rule' to the 'times' at Daniel 7:25 or Daniel 12:7.