Add n days to an existing date

// Write a function that takes as argument a date instance (a) and a number (b)
// It should add b days to a and return the number of milliseconds since January 1, 1970, 00:00:00 UTC
function
myFunction
(
a, b
)
{

return
}
Test Cases:
myFunction(new Date(Date.UTC(2000,01,01)), 31)
Expected
952041600000
myFunction(new Date(Date.UTC(2000,01,01)), 10)
Expected
950227200000
myFunction(new Date(Date.UTC(2000,02,28,)), 2)
Expected
954374400000