TTim: validate output times

[Huite]

No output can be computed at t=0 for a given change. We could:

• error if the output time falls on a time where a change occurs.
• shift the output time by tmin, guaranteeing that any time falls at least tmin after any change.

The first is somewhat tedious (you have to shift output times).
However, the second is magical, so erroring is probably the best option.