In which order are these applied?

*Blaspemous Tendencies: +15 to Warp Navigation Tests. All crew suffer -5 WP.
*Tempermental Warp Engine: Warp journeys take an additional 1d5 weeks 60%, or less 1d5 weeks 40%. Occasionally gets Lost in Space.
*Runecaster: +20 to Warp Navigation Tests. Any journeys take half normal time.
*Markov 1 Warp Engine: Reduce base Warp travel times by 1d5 weeks. May be further modified by Navigation (Warp) Tests.
*Starchart Collection (Best): Reduce Warp travel times by 1d5+5 days (to a minimum of 1) for warp travel. +50pt on Exploration endeavour.
*Warpsbane Hull: +10 to Warp Navigation Tests, choose re-roll on the Warp Encounters Table
*Warp Navigation Tests

What happens if I have a base travel time of zero and then get 2 degrees of success on the Navigation Charts? (ie. What happens if my Warp Drive Divides by Zero). Does Tzeentch appear with an Error Message?

How do you handle players who deliberately try to engineer a Time Travel effect? Advice on running Time Travelling campaigns?

Traveling back in time during a Warp voyage is something that should only happen very rarely. I rule that the travel time is never less than a few hours no matter how good the ship/Navigator combination is. Appearing before you depart can happen, but in play it should only occur when the GM wants it to, not be something the PCs can try and make happen by taking a short trip in a ship with lots of base time reductions.

Any other way of handling it probably leads to an unplayable game ending in either the Inquisition confiscating the ship or the Chaos Gods laughing as the accumulated paradoxes tear the fabric of reality apart.

What I do is declare that 1 day is the minimum base travel time. A good Navigator roll can then turn this into 6 hours at the shortest.

By the rules, cannon, and fluff arriving someplace before you left can and does happen.

I allow it with the following restrictions.

1. A player/ship cannot be 2 places at the same time within the same light horizon (see number 2 for the exception). Clear as mud? Basically it is engineered so that the time travelers can never see themselves. Lets say a ship is going to make a jump to a location 1 light week away, but he arrives 1 day before he left. This is fine and I allow it as his 1 day light horizon is less than distance traveled. Now lets say that instead of 1 day he arrived 2 weeks before he left. This is no good as the 2 week light horizon is greater than the 1 light week of distance he traveled. I handle it this way. I roll a 50/50 chance for either changing the arrival time, or changing the distance. In the above example he would either arrive 6 days 23 hours and 59 minutes before he left, or he would overshoot his destination by 1 light week (so that he arrived 2 weeks before he left but was 2 light weeks away from his starting location). You also have to figure in the light horizon he has from his starting location as well (which was not dealt with in the example.

2. Two instances of an individual CAN be in the same light horizon, but only by GM fiat. It never happens by accident and must be planed out ahead of time. This is basically done so that gameplay will not be messed up.

It actually ends up being easier to keep track of in practice than you would think.

My players actually tried to hatch a plan to travel back and prevent the Horus Heresy. (It was nixed due to staggering paradox, since thier Warrent of trade was earned slightly before the heresy.)

What happens if I have a base travel time of zero and then get 2 degrees of success on the Navigation Charts? (ie. What happens if my Warp Drive Divides by Zero). Does Tzeentch appear with an Error Message?

A divide by 0 situation is when you try to calculate x/0. What you are talking about is 0/x, which has an answer of 0.

How do you handle players who deliberately try to engineer a Time Travel effect? Advice on running Time Travelling campaigns?

If the players are lucky, the attempt simply fails. If not, it fails badly.

I'm not aware of any instance of deliberate time travel in 40k, or even how one would go about trying.

Bilateralrope said:

The Ordo Chronos blurb heavilly implies that attempts have been made at time travel, and the Inquisition, apperently, has a time travelling ordo to prevent it. (A vague mention of tech and deliberate attempts suggest that the AdMech may have an STC time machine?)

'Hello, I'm The Inquisitor."

However, the entire Ordo Chronos then disappeared mysteriously, which would suggest that, even if time travel is possible in 40k, it's probably not stable and will bite you in the arse.

BaronIveagh said:

Ordo Chronos have all disappeared, though. Vanished without a trace. My personal theory is that someone went back in time and removed the first leader of the Ordo.

My theory is they were all killed in the last Great Time War.

EXTERMINATE

MILLANDSON said:

Or they've gotten better at hiding. Or have already won.

But I doubt time travel in 40k is ever likely to end well for those involved.

Bilateralrope said:

This would be my take. In the game I'm currently playing, we frequently arrive before we leave (we have a tempermental warp engine and otherwise tend to make good time), but the ruling was simply that any attempt to backtrack will always result in our having been gone at least as long as our relative time, the warp just doesn't allow easy time travel. Any attempt at astropathic messaging just gets "temporally adjusted" by the warp, in a similar fashion.