Good Point about the Eternal Now and unnecessary anachronisms, but Harry Potter and Ranma 1/2 may not have been the best examples for it.
Rowling plays rather fast and loose with the dating of her story herself. Is the Prime Minister in HBP Tony Blair or John Major? The start of term is always the same date and weekday and any attempt to make a coherent timeline is doomed from the start. So anybody who has something like easy internet access and widespread cellphone usage or 2 Pound coins or the Millennium Wheel for their HP story has an excuse even if it should not have been around in the early nineties.
Ranma on the other hand is almost timeless. The manga especially could easily be moved back or forwards for decades without any major conflicts with canon. There is almost no mention made of technology thoughout the series that would require it to be dated at a certain point. What little there is of fixed dates is really minor and irrelevant to the plot at large. If a writer wants to give Nabiki an I-Pod that really would not count as major violation of canon.
But there are other fandoms who take much greater care with their timelines and their chronolgical setting where any sort of anachronism will have a much greater and far more jarring impact. If for example the ubiquitous presence of cell phones would have made several canon plots about people trying to contact each other or somebody searching for a pay-phone unlikely it gets harder to simply set a fanfic in the 'present'. If the coming of the millenium was a major plot point you can't really have people walking around today without having aged a few years.
It should also be noted that many fandoms are set canoical in a sort of "Eternal Now" especially the more succesful ones that go on and on without having the charcters age too much, but still keep involving current elements. Others have a built in alternative history because the creators could not easily adapt their stories to major real live events that occured during production.
The most problematic are fandoms that are set in a timeline that must have diverged from our own at some point. They usually occur when you start out with a sci-fi series set in a distant future like the year 2000 and then reality catches up with them. Some handle that better than others compare for example the UN War from Macross and the Eugenic Wars from Star Trek that should have happened a while ago.
In those divergent timelines you in theory should pay attention to avoid anachronisms too. Don't write an Evangelion fic that has characters do things like referring to 9/11 or use technology that never got invented. Shinji owns an S-DAT player with a magnetic tape and not an MP3 player with memory sticks for a reason.
But while it is bad thing to make such 'mistakes' for no reason at all, there is no reason to hold back if there is a good reason to introduce any anarchonisms. Crossovers almost always require some 'adjusting' of timelines to work if the different series you are crossing are supposed to be set in the same reality. And if your 'one unicorn' that you add to canon requires the story to be set in a differnt time than canon indicates then that is certainly a good excuse and not an unnecessary anachronism.
Rowling plays rather fast and loose with the dating of her story herself. Is the Prime Minister in HBP Tony Blair or John Major? The start of term is always the same date and weekday and any attempt to make a coherent timeline is doomed from the start. So anybody who has something like easy internet access and widespread cellphone usage or 2 Pound coins or the Millennium Wheel for their HP story has an excuse even if it should not have been around in the early nineties.
Ranma on the other hand is almost timeless. The manga especially could easily be moved back or forwards for decades without any major conflicts with canon. There is almost no mention made of technology thoughout the series that would require it to be dated at a certain point. What little there is of fixed dates is really minor and irrelevant to the plot at large. If a writer wants to give Nabiki an I-Pod that really would not count as major violation of canon.
But there are other fandoms who take much greater care with their timelines and their chronolgical setting where any sort of anachronism will have a much greater and far more jarring impact. If for example the ubiquitous presence of cell phones would have made several canon plots about people trying to contact each other or somebody searching for a pay-phone unlikely it gets harder to simply set a fanfic in the 'present'. If the coming of the millenium was a major plot point you can't really have people walking around today without having aged a few years.
It should also be noted that many fandoms are set canoical in a sort of "Eternal Now" especially the more succesful ones that go on and on without having the charcters age too much, but still keep involving current elements. Others have a built in alternative history because the creators could not easily adapt their stories to major real live events that occured during production.
The most problematic are fandoms that are set in a timeline that must have diverged from our own at some point. They usually occur when you start out with a sci-fi series set in a distant future like the year 2000 and then reality catches up with them. Some handle that better than others compare for example the UN War from Macross and the Eugenic Wars from Star Trek that should have happened a while ago.
In those divergent timelines you in theory should pay attention to avoid anachronisms too. Don't write an Evangelion fic that has characters do things like referring to 9/11 or use technology that never got invented. Shinji owns an S-DAT player with a magnetic tape and not an MP3 player with memory sticks for a reason.
But while it is bad thing to make such 'mistakes' for no reason at all, there is no reason to hold back if there is a good reason to introduce any anarchonisms. Crossovers almost always require some 'adjusting' of timelines to work if the different series you are crossing are supposed to be set in the same reality. And if your 'one unicorn' that you add to canon requires the story to be set in a differnt time than canon indicates then that is certainly a good excuse and not an unnecessary anachronism.