UserAgent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Ubuntu Chromium/48.0.2564.116 Chrome/48.0.2564.116 Safari/537.36

Steps to reproduce the problem:
1. Run Chromium in the UK timezone, for instance with:

     $ TZ=Europe/London chromium &

2. In the F12 JavaScript console enter each of these in turn:

     new Date(1971, 0, 1).toISOString()
     new Date(1971, 6, 1).toISOString()
     new Date(1972, 0, 1).toISOString()
     new Date(1972, 6, 1).toISOString()

What is the expected behavior?
Both the 1971 dates should be in UTC+1 (‘British Summer Time’), so in UTC should show as 23:00 the previous day:

  "1970-12-31T23:00:00.000Z"
  "1971-06-30T23:00:00.000Z"

1972-01 should be in UTC, and 1972-07 in UTC+1:

  "1972-01-01T00:00:00.000Z"
  "1972-06-30T23:00:00.000Z"

What went wrong?
The dates in 1971 are treated as though they were in UTC, not UTC+1:

  "1971-01-01T00:00:00.000Z"
  "1971-07-01T00:00:00.000Z"

The 1972 dates are correct:

  "1972-01-01T00:00:00.000Z"
  "1972-06-30T23:00:00.000Z"

Most years Europe/London is in UTC during the winter and UTC+1 during the summer. However, for the 3 winters of 1968–71 it remained in ‘daylight saving’ UTC+1 throughout the winter, and stayed in that timezone during the summer.

So the 1971 dates should be ‘23:00 the previous day’ in UTC, just like the 1972-07 date is.

That the two 1971 dates use the same UTC offset as each other suggests that you are treating that year specially, but the wrong way round: you have it as winter throughout the year rather than summer all year.

Did this work before? N/A 

Chrome version: 48.0.2564.116  Channel: n/a
OS Version: 
Flash Version: Shockwave Flash 14.0 r0

See this Firefox bug for the background and why this matters:
https://bugzilla.mozilla.org/show_bug.cgi?id=994019#c33

Firefox gets the UTC offsets right. Unfortunately that breaks axis labels in Google Books Ngram Viewer, which itself has a bug. The Ngram Viewer bug doesn't occur in Chromium because of this bug.

I reported the Ngram Viewer bug as Google Books feedback (I couldn't find Ngram Viewer-specific contact details), so hopefully they will change their code not to rely on this Chromium bug.