Nother/Timeslip effect
A timeslip effect is a magical effect often observed when races with magical ability begin to colonize planets. When an event that would normally cause a change of Age of Magic occurs, this effect causes the planet to be displaced temporally; generally, to some point in the past. A second such event may restore it to its original time; further changes are not observed with subsequent events. The ultimate cause of timeslip effects has not been determined.
The first observed timeslip effect affected the planet Mars, which was first colonized in the late 20th century. Earth lost contact with Mars at the beginning of the year 2000, an event which coincided with various other events which collectively came to be called the Millennial Shift. When Mars reappeared in 2005, it had been terraformed and its inhabitants reported that five hundred years had elapsed locally.
That such timeshifts would be frequent, however, was not suspected until several centuries later when faster-than-light communication was invented. The first message received by ansible was a distress signal in Chinese from a then-unknown planet, which reported being colonized several hundred years before Humans had even begun to harness electricity. The ansible's inventor determined that this colony had experienced a similar effect to that seen on Mars, and communication with several other inhabited worlds was made. The timeslip effect was so frequently observed that it came to be the assumption that any extrasolar settlement could be traced to a future Earth colony, until the late 26th century when true alien intelligences were first encountered.
[There are several issues with causality that need to be worked out for this—in particular, future settlements would have more advanced technology and access to future information; would anything keep them from sharing this with Earth &c., or what would be the effects if not? Note that it's already determined that further settlements have more advanced technology than Earth, but this is in large part due to backwardness on Earth's part, and wouldn't affect Mars or Luna.]
