Why would that be difficult to accept? We would get the very same thing if we tried to carbon-date something, cross-checked against our tables, and came up with the odd fact that the carbon in the sample has decayed less than if the object were created today. There's a mathematical method there that yields negative numbers in certain unlikely cases, one of these being time travel.
As for the probe, we learn that it time-traveled, for whatever reason. It stands to reason, then, that the eventual weapon would time-travel, too. Since Earth doesn't do time travel (yet!), there's no harm in trying out a little test version in 2151 and then following up on that with the Armageddon version in 2150. Or 1732 BC for that matter.
Timo Saloniemi
As for the probe, we learn that it time-traveled, for whatever reason. It stands to reason, then, that the eventual weapon would time-travel, too. Since Earth doesn't do time travel (yet!), there's no harm in trying out a little test version in 2151 and then following up on that with the Armageddon version in 2150. Or 1732 BC for that matter.
Timo Saloniemi