Ancient time device
A long, slender prototype of Ancient construction, designed by the Ancient Janus. The unit is shaped to easily fit in the rear compartment of a Puddle Jumper. Designed to transport the entire ship through time, the experimental technology was banned by the Atlantis council of the Ancients. They forbade Janus from continuing his work on the grounds that the timeline would be abused with the use of the device. With the possibility of any number of causalities, the ramifications of temporal travel were too horrifying to ignore.
When the Atlantis expedition originally traveled to the city power was not sufficient to sustain the shield that kept out the ocean. John Sheppard, Elizabeth Weir and Radek Zelenka became trapped within the Puddle Jumper with the time device. When the Jumper bay flooded, Sheppard used the time device (inadvertently) to free them. Later, Dr. Weir was sent back in time during the first siege of Atlantis, proving to Janus and the council that his technology worked.
Another time device, presumably constructed by Janus, lay in hiding on Arkhan's planet, where the ship had been abandoned. This Puddle Jumper was eventually used to take SG-1 back to ancient Egypt to steal a Zero Point Module from the Goa'uld Supreme System Lord, Ra. Unfortunately, the Ancient high council's worst fears were realized, as when SG-1 was discovered they were stuck in the past, and changed the timeline so that their future selves were altered.
Fortunately, the altered members of what once was SG-1 was able to return into the past and correct the causality, forcing history to play out as it had originally (with Ra leaving the Stargate on Earth).
Janus applied a secondary power pack to the Puddle Jumpers that were equipped with the time devices. In the event that the Puddle Jumper's primary energy matrix had run dry, the secondary power pack could be disconnected from the time device and rerouted to power the Jumper instead.
It's Good To Be King - SG-1 unearths an Ancient Puddle Jumper with a time device within, but are not immediately certain what the component in the rear of the ship does.
Before I Sleep - The Atlantis expedition learns of their initial fate, and how three survivors managed to escape in a Puddle Jumper hooked into a time device.
Moebius, Part 1 - Eager to aid the Atlantis expedition with Z.P.M.s, SG-1 embarks on a dangerous plan to use the time device and recover a module in the past -- when it was fully powered.
Moebius, Part 2 - SG-1, in an altered present, is forced to use the time device in the hopes of repairing what went wrong in ancient Egypt.