Recency compare function (JavaScript)

Recency.js

// a, b: Date-time value (null if unknown or "never")
// altCompare: An alternative comparison function(a,b).
function recencyCompare(a,b) {
    if (!a && !b) return altCompare(a,b);
    if (!a) return 1; // 'b' has an error and 'a' doesn't
    if (!b) return -1; // 'a' has an error and 'b' doesn't
    if (a == b) { return altCompare(a,b); }
    return a < b ? 1 : -1;
}