Reroll 1s and -1 to hit.

[Beatnik cryptek]

OK, say a triarch stalker firing at a unit allows necron units firing at it in the same turn to reroll hit rolls of 1.

 

Now I'm firing at a unit that has a -1 to hit and I roll a 2. The -1 makes that a 1, do I get to Reroll it?

[Eddie Orlock]

No, if you check your sequences in the core rules you alway check for re-roll conditions before you apply modifiers.

[CaptainMarsh]

Eddie is correct. Re-rolls come before modifiers.