Air dates listed in IMDb....?

It ends up in a way being largely attributable to the series itself, as a whole, being "all over the map", both figuratively and literally. A problem with putting dates on UK TV series is that they don't have a single nation-wide broadcast, unlike the US. "London" is generally adopted as the standard, but even that wouldn't work for the David Keel episodes, as nine of those episodes were never shown there. "Granada" becomes the standard for that season. As it is, there were two interruptions of the season, so you don't get a smooth weekly sequence in any event. In later seasons, you find the London schedule having an odd tendency to shift days of the week. Bearing all this in mind, and using the information from the "Avengers Forever" website, which claims to have thorough research behind its Granada and London dates, the following corrections apply to IMDB's airdates from the "Episode Cast" page, for the David Keel through Emma Peel ("from Keel to Peel") episodes:

S1E1 "Hot Snow" should be Jan 7, 1961
S4E2 "The Gravediggers", to S4E13 "Too Many Christmas Trees" should all be two days earlier
S4E18 "The Thirteenth Hole" and the rest of S4 should all be one day earlier
S5, all 16 episodes, should be two days later, except E14 "Something Nasty in the Nursery" and E15 "The Joker" should be one day earlier
S6E1 "Return of the Cybernauts" to S6E5 "You Have Just Been Murdered" should all be two days earlier
S6E6 "The Positive Negative Man" to S6E8 "Mission...HIGHLY Improbable" should all be one day earlier

Beyond this point (the Tara King episodes), London, indeed, the UK itself, ceases to be the primary standard, as the episodes mostly aired in the US before the UK -- the series, ever since switching to colo(u)r, had been financed by its American network, coincidentally also called ABC like the British company that produced the show. The date IMDB lists for the first of these, "The Forget-Me Knot", already reflects this with the date March 20, 1968. In the complete list of Tara King episode 'world premieres', in that order, the dates are shown here for US broadcast, except in a few noted cases where, owing to different ordering of episodes in each country, a few *London dates prevail:
The Forget-Me-Knot March 20, 1968
Invasion of the Earthmen March 27
The Curious Case of the Countless Clues April 3
Split! April 10
Get-A-Way! April 24
Have Guns —— Will Haggle May 1
Look — (stop me if you've heard... May 8

Game Sept 23
Super Secret Cypher Snatch Sept 30
You'll Catch Your Death Oct 7
Noon Doomsday Oct 28
Whoever Shot Poor George Oblique... *Oct 30
Legacy of Death Nov 4
False Witness *Nov 6
They Keep Killing Steed Nov 11
All Done with Mirrors *Nov 13
Wish You Were Here Nov 18
The Rotters Dec 16
Killer Dec 30
The Interrogators *Jan 1, 1969
My Wildest Dream Jan 6
The Morning After *Jan 29
Love All Feb 3
Take Me to Your Leader Feb 10
Fog Feb 17
Stay Tuned Feb 24
Who Was That Man I Saw You With? March 3
Pandora March 10
Homicide and Old Lace March 17
Thingumajig March 24
Requiem March 31
Take-Over April 14
Bizarre April 21