2020 seems to be the most likely date according to this article in "The Guardian".

"Digital radio switchover was once mooted for as early as 2015, but the government has indicated that radio stations are unlikely to follow television's lead by switching off their analogue signals until at least 2020.............

Paul Keenan, chief executive of Bauer Media, owner of Kiss, Magic and digital-only station Planet Rock, said: "There's more we could do to accelerate things. It would be good if the BBC put Radio 1 and Radio 2 exclusively on DAB for example.""


Meanwhile "Ariel" has a report:-

"More money is to be invested in digital radio as the industry gears up to switch off FM transmitters within the next few years.

Communications Minister Ed Vaizey said up to £21m will come from the BBC, government and commercial radio."

It is the increase in the number of audio listening platforms that is  probably causing the FM closure doubts. In addition the younger listener is rapidly discarding standard radio listening in preference to the growing number of music streaming sites, MP3 internet apps etc.
I think the powers that be are secretly giving up on DAB, It hasn't worked and is a dead duck as far as marketing/PR goes. Most importantly it is still very unavailable in rural areas. Over 90% coverage says the official blurb---what absolute tosh!
Much as I would love to go out and buy a fantastic DAB portable radio, there is little point in the Stevenage area until the reception is good enough to receive signal in ALL the rooms in my house and not just in one bedroom. As for the option of having DAB in the car-I would not even bother turning it on--it would be unreliable.
Meanwhile, with rubbish FM reception in this area too, I am quite happy with radio obtained via Freesat through the TV downstairs(fine with a Bose speaker added!), plus increasing reliance on radio via internet, smartphone and tablet.
For interest, my listening preferences are
1) Radio 4 today programme(Iphone)- 20 mins a day
2) Spotify music streaming and Itunes via broadband laptop and Iphone- 2 hours
3) Radio 2 output-via Iplayer and Spotify ( using the new Playlister facility) , specialist programmes ( not the daily strip shows) -4 hours a week.
4) FM in the car-whatever is available and suits the mood-very good BBC reception all over the country-usually R2 but R4 if calming talk is needed. If I have time I select a self compiled CD of music.  

Incidentally -who on earth listens to Podcasts these days and why on earth are they still going?.....dump 'em. ...and as for local radio-ok for power cut news and road closures during floods/snow but that's about all I reckon!

I'm not sure that statement is strictly true, Dickie. It's really the ILR stations that are pushing for it (through the Digital Group) because they want to get back to a single transmission medium. Having two distributions (analogue FM and digital DAB) to worry about is costing them buckets of cash - and they're not that flush nowadays!
The Beeb is, I believe, just going along with it for the moment ... after all they started the DAB ball rolling. But they did it too early, and as such the technology being used (MPEG-2/DAB) is now relatively old (20 years!) in comparative terms. If they were to start again now, then I believe they'd be using DAB+ which is much more efficient in coding terms, and they'd actually have a better output than the MPEG-2 stuff they're currently using. [DAB+ is very much akin to iTunes, in that it's based on using AAC (aka MPEG 4), at lower bitrates than MPEG-2 for the same quality.] Still limited bandwidth of course.
But I agree that the standard quality of DAB is nowhere near that of the present FM chain - and I'm saying nothing more about the lack of coverage out here in the sticks! It's simply dreadful, and I don't for a moment believe the claptrap about raising Tx powers to give better coverage, or the spending of millions on more transmitters. Have they not considered the power all those extra Txs will be using?

I wrote to my MP a couple of years ago, asking him to bend the ear of Jeremy Hunt (who was the Minister responsible for the decision at that time!) and beg him NOT to push the DAB changeover at that juncture. Fortunately (and seemingly contrary to his natural bent) he decided that DAB wasn't widely available at that time, and he kicked the can down the road for a few years. But one thing does seem to have made them pause since then and that is that someone gently pointed out to them that the FM band isn't really of any value to anyone except broadcasters (i.e they wouldn't get much external money if they tried flogging it off) and since the broadcasters are already using it ...
I did think there was a glimmer of 'new light' a couple of years ago, when Rupert Brun arranged for a 224Kb/s AAC transmission of the Electric Proms be made available over the internet. It was so much better than the DAB tx ... but sadly there's been no repeat of such a trial.

IMHO the only real hope for a 'digital future for radio' is that the DAB/DAB+ chipset becomes so cheap that those who are making digital radio sets (PURE) find it beneficial to use them. Then it would be possible to switch overnight to DAB+ without members of the public having to buy newer radios! But I'd stress I still want/prefer to have FM stereo available until that situation were to exist.

Roundabout - That's quite likely!
You didn't specify which network you were listening to, which is relevant because different networks are allocated differing bit-rates, and it's that factor which ultimately defines how well (accurate) the digital encoding represents the original analogue signal.
If they don't allocate enough bits to that service then the HF content gets reduced [this is the case with R4, which is mainly speech-based and therefore deemed to have less HF than R3 for instance] so that the digital representation is most accurate over the main audio frequencies which people hear.
It 'used to be the case' that R3 was allocated 256kb/s but then that dropped to 224kb/s, and R4 was variously allocated between 128 and 160kb/s (for speech or musical content). WS was reduced to 64kb/s (and had no top whatsoever!) - although to be more technically accurate the allocation is done in Capacity Units (CU) since that allows for the inclusion of error correction code as well.

When R&D first contributed to the Eureka-147 technology (on which DAB is based) it was reckoned that MPEG-1 Layer 2 (known as MPEG-2 or MP2) required at least 256kb/s to render audio deemed to be "indistinguishable" from the original. Sadly, there simply aren't sufficient 'bits' available to allocate that high bit-rate to all the services which are put onto DAB, so they profile the services according to the content. [Just for information, with MPEG-4 (aacPlus) DAB+ requires just 128kb/s to render the same "indistinguishable" audio quality: such has been the increasing efficiency of coding technologies over recent years.]
Such a shame that DAB is not forward compatible with DAB+.

Thanks Bill and John. what you say conclusively shows that DAB as operated at the moment is an accountancy led sham. There is no doubt that R4 suffers a lot and you have explained why it is so variable in quality. With all it's reception challenges FM seems the better way.