In general, yes, it is pointless to add times. But I wouldn't call it meaningless.
For extremely large values of "in general." The only edge case where it appears to be meaningful is the one you pointed out, where one of your endpoints is the origin of the scale. If you had started Deep Thought at literally any other point in time, the addition would fail.
The addition wouldn't fail; it would just not be very useful. Interval scales do support addition, which comes in handy when you want to, say, calculate the center of mass for interesting events.
What would fail would be trying to determine the factor by which 2010 is later than 1666.
For extremely large values of "in general." The only edge case where it appears to be meaningful is the one you pointed out, where one of your endpoints is the origin of the scale. If you had started Deep Thought at literally any other point in time, the addition would fail.