Time#-

static VALUE
time_minus(VALUE time1, VALUE time2)
{
    struct time_object *tobj;

    GetTimeval(time1, tobj);
    if (IsTimeval(time2)) {
        struct time_object *tobj2;

        GetTimeval(time2, tobj2);
        return rb_Float(rb_time_unmagnify_to_float(wsub(tobj->timew, tobj2->timew)));
    }
    return time_add(tobj, time1, time2, -1);
}