Soo. People finally found No Time To Explain. 

it was time locked like the sleeper but with way less anticipation

here are the steps on how to get it (I think the quest starts after you find the secret ending in today’s heroic mission)

the perks are 

  • Full Auto
  • Speed Reload
  • Hand Loaded
  • Fitted Stock
  • Rewind Again

Exotic perk

Rewind Again: Precision hits are immediately returned to the magazine

the last picture is from the comment in the second link