Problem about Check for leap year

[ysgtool]

Hi:
I learn programming only a few time.
When I use Date.pase() to check if a datestring is valid. I found it can't check leap year correctly . So I view some content of the date-utils.js file. I find there are some places with the code like (month === n) where the month is a string and n a number. I wonder if their type need to be converted the same. So I ask some help.
Please forgive me for my bad english.
Thanks!