Efficient MySQL Date Verification in Javascript?

I'm not the best person I know at determining what is efficient in JavaScript (ECMAScript) though I would like to think that this could help someone.

/**
 * Make sure that the passed value is valid for the proposed condition. If
 * isRequired is true, dateString must not be blank or null as well as being
 * a valid date string. If isRequired is false, dateString may be blank or null,
 * but when it's not, it must be a valid date string. A valid date string looks
 * like YYYY-MM-DD
 *
 * @param dateString {String}
 * @param isRequired {Boolean}
 * @returns {Boolean}
 */
function isDateValid( dateString, isRequired ) {
    var regex = /^\d\d\d\d-\d\d-\d\d$/ ;
    var retVal = true ;

    if ( ! isRequired ) {
        if ( ( null == dateString ) || ( '' == dateString ) ) {
            return true ;
        }
    }
    else {
        retVal = ( ( null !== dateString ) && ( '' !== dateString ) ) ;
    }
    retVal = ( retVal && ( null !== dateString.match( regex ) ) ) ;
    if ( retVal ) {
        var daysInMonths = [ 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 ] ;
        var yr = parseInt( dateString.substring( 0, 4 ) ) ;
        var mo = parseInt( dateString.substring( 5, 7 ) ) ;
        var da = parseInt( dateString.substring( 8, 10 ) ) ;
        if ( ( yr % 4 ) && ( ( yr % 400 ) || ! ( yr % 100 ) ) ) {
                daysInMonths[ 1 ]++ ; // Leap day!
        }
        if  ( ( yr < 2000 ) || ( yr > 2038 )
           || ( mo < 1 ) || ( mo > 12 )
           || ( da < 1 ) || ( da > daysInMonths[ mo ] )
            ) {
         retVal = false ;
        }
    }
    return ( retVal ) ;
}

If you know of a more efficient way to handle a MySQL (YYYY-DD-MM) date validation, please reply to this post. :-)