Hammerboy217: Underboss:

i got it...

you know that crystal like thing that made slip space all funky looking? u know where the covenant ship fired a torp and it went all around?

sent them back in time. thus linking to the guys question...

well thats my theory anyways...

The crystal had space-time bending capabilities, thats why John and company were able to get back to reach within that time limit, and why they were able to get to Eridanus Eridani, where the rebel base was located.  And once again, it was shown that when Corporal Locklear destroyed said crystal, it was squeezed out of Gettysburg-Ascendant.