A time source isn't required, a clock is. Further, if the origin server does not have a clock, any proxy (such as HAProxy) is still required to add the Date he

A time source isn't required, a clock is. Further, if the origin server does not have a clock, any proxy (such as HAProxy) is still required to add the Date header if it has a clock, as if it were the origin server. In practice, there are very few functional systems without clocks.