you realize that a single '=' is an assignment, not a comparison? so you are trying to assign the value of "true" to what you got back from your method call...which is just wrong.
You are also assigning the value of 2 to month, and then trying to use the "2" in a boolean expression.
Also, you should never compare a boolean to "true" or "false", you just use it. You probably need something like:
if (month==2 && isLeapYear(_year) && day<=1 && day>=29)
There are only two hard things in computer science: cache invalidation, naming things, and off-by-one errors