Arnold Zwicky in Language Log quotes a striking pair of sentences from a Palo Alto Daily News story:
This was the first officer-involved shooting in San Mateo since Labor Day, when a homeless man wielding a knife was shot. The last such shooting in the city since that incident was almost 24 years ago, Raffaelli said.
As Zwicky says, “In the first sentence we have an ordinary use of temporal since… But in the second sentence the time span is between an anchor time (again, last Labor Day) and an EARLIER time (of the shooting 24 years ago). Time seems to be running backwards; before, not since, is the appropriate P (preposition or subordinator) here.” He analyzes it as a deliberate usage (“The writer seems to have generalized since from referring specifically to elapsed time… to referring to any span between two times”), but it seems to me much more likely that it was an accidental result of botched copyediting and that all parties involved, if shown the sentence as printed, would say in annoyance “How did that slip through?” It just seems too far outside the norm to be anyone’s actual usage.
But I’ve been wrong about such things plenty of times, so I ask the assembled multitudes: does it seem likely to you that someone in the normal course of speaking or writing English could use “since then” to mean “before then”?