Took a flight to Shobdon yesterday, 26 April, and both the SD Pilot Log and AFE Plate (downloaded yesterday) showed the AFIS frequency as 118.115.  This does not come into effect until 1 May 2018. Fortunately I had the current frequency, 123.500, to hand.
I believe that  a similar thing happened‌‌ with a change at Oxford, in that the "new" frequency was provided before it came into effect?

This is going to happen a lot this year as‌ ground stations change to 8.33 frequencies.

I don't kn‌‌ow what triggers the change in SkyDemon? If it is not possible to relate a frequency to the date of flight, would it be possible to show two frequencies, with a "wef Date" for the new frequency?‌‌
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Unfortunately not. It isn't always going to be possible for us to precisely align frequency changes to dates because some of the 8.33 switchover dates are not aligned with the aeronautical AIRAC cycle. This mess isn't our fault and unfortunately we don't have the resources to build in extra functionality to attempt to mitigate it for you.

OK, at least we know. Quite agree that it is a mess, not of your doing. Shows that the AIRAC cycle is not fit for purpose in many circumstances.