Adjust test value so that timezone has no effect

If the test is run in a timezone that is behind UTC it fails because
the time generated is ahead of 0000-01-01 00:00:00. Just increase the
time subtracted so that timezone has no effect.
docs, Cookie values are String based. Closes #12860. [ci skip]
Rely on backticks instead of tt tags [ci skip]

Since the language in code blocks is inferred, if the code contains tt
tags, the block will be parsed as XML for instance while it is Ruby.