According to MDN - parsing of date strings with the Date constructor (and Date.parse, they are equivalent) is strongly discouraged due to browser differences and inconsistencies.
I suggest to use this style
defaultDate: new Date(2016 , 4, 26, 0, 0, 0),
As for string representation it should be in RFC2822 or ISO 8601 format according to JS documentation.
defaultDate: new Date('2016-04-26T00:00:00')