isValidDate.js

function isValidDate(day) {
    return !(this.earlierLimit && day < this.earlierLimit || this.laterLimit && day > this.laterLimit);
}

// O, why thou not use variables?!!
function isValidDate(day) {
    var early = this.earlierLimit && day < this.earlierLimit,
        late = this.laterLimit && day > this.laterLimit;
    return !early && !late;
}