I believe it's either 2017-2018 or just 2018.
The Nerdist has a page reviewing the DVD of that season. They claim 2018. This would make some sense, since 1960s popular science fiction series often tended to go for future times which were a round number of years in the future, such as 50 or 100. If this thinking is right, both 2017 and 2018 are possible as the season wore on, seeing that it started in late December.
Here is part of the script at http://www.chakoteya.net/DoctorWho/5-4.htm
The only occurrence of "2018":
DOCTOR: A very timely and welcome rescue.
ASTRID: Don't speak to me for a moment, please.
(The Doctor looks at the permit posted in the back of the helicopter - Australasion (sic) Zone. Type Helicopter, (private). Valid until 31st Dec 2018. Class Reg No L23X. Victoria has her arms around Jamie's neck.)
JAMIE: But what is this thing, Doctor?
DOCTOR: It's a helicopter, Jamie.
DOCTOR: A chopper. You know, a whirly bird.
JAMIE: He say's it's a bird.
DOCTOR: No, no, no. It's a primitive form of flying machine.
VICTORIA: Well, at least we're safe now, aren't we?
ASTRID: Depends on what you mean by been safe. They've shot a hole in the fuel tank. We might blow up any minute.
(Rod gets Curly back to the hovercraft, then they head back onto the water and follow the helicopter as far as they can. Astrid flies inland to a lone bungalow.)
ASTRID: Here we go.
(She lands the helicopter. They all get out, Astrid last of all, holding her left arm.)
The only occurrence of "2017":
[Underground control room]
SALAMANDER: Not now, we are too busy.
SWANN [OC]: This is urgent.
SALAMANDER: Oh very well.
(Swann enters with the piece of paper.)
SALAMANDER: Now what's the matter, huh?
SWANN: What's this? What is this?
SWANN: Newspaper. It's a piece of newspaper.
SWANN: Well, go on, look at it. Read it. Last year's date. Look at the bit of headline there. There!
(Holiday liner sinks, many feared lost. Dated Friday August 16, 2017.)
SALAMANDER: Control yourself, Swann. You'll go mad.
SWANN: Holiday Liner Sinks! You say there's a global war, radiation everywhere. How can there be holiday liners? Tell me how! You've lied to us, haven't you? Haven't you!