Fixes incorrect dates from parsed values

[maxf]

Closes #23

Using new date() returns the current system date. Modifying the date afterwards can have unexpected consequences if the system's date is the 31st. For instance, parsing 2015-09-12

var date = new Date()  # date is set to system date (2017-05-31, for instance) 
date.setUTCFullYear(dateParts[0]); # date is now 2015-05-31 
date.setUTCMonth(dateParts[1] - 1); # date is now 2015-10-01, because 2015-09-31 is not a valid date
date.setUTCDate(dateParts[2]); # date is now 2015-10-12

The resulting date is one month ahead of what was parsed.

Writing a regression test is difficult as it would necessitate setting the system date.

[bwindels]

Look good, thanks for the fix!