The error message looks like it makes sense, it's trying to parse the input as "2006-01-02T15:04:05Z07:00" (with the double quotes) and it chokes on missing the leading ".

I guess it could use fmt.Sprintf("%q", s) instead to get "\""? Though I think that would actually make the parse error less clear as you now have to undo the quoting to get the original input.

The time package cannot use %q (strconv.Quote), but a simple improvement would be to use backticks to quote strings that contain double-quotes.

Change https://golang.org/cl/307192 mentions this issue: time: use backticks to quote strings that contain double-quotes

For the record, the fix in 1271e9a caused some of our tests that passed with Go 1.16 to fail with Go 1.17 because they were written like this:

{
	description: "message with invalid timestamp",
	message:     plainMessage(`{"Value":42,"Timestamp":"HALLOJ"}`),
	expectationOnError: errtest.And(
		errtest.HasMessageContaining(`Unknown message format: parsing time ""HALLOJ""`),
		errtest.HasMessageContaining(`cannot parse "HALLOJ""`),
	),
},

Test output:

Error message should contain

  Unknown message format: parsing time ""HALLOJ""

Actual error:

  Unknown message format: parsing time "\"HALLOJ\"" as "\"2006-01-02T15:04:05Z07:00\"": cannot parse "HALLOJ\"" as "2006"

Not meant as a criticism, just a side note that perhaps someone may find interesting. 🙂