Similar to #8424, Jekyll::Utils is a module as well.
Instance variables should be used carefully since modules can be mixed-in into various classes.

Would like to see your profiling / benchmark results. Is Time.parse(input) slower or Time.parse(input).localtime ?
If it is just Time.parse(input), then we could just cache the result of Time.parse(input) and call :localtime on it to get a (mutable) non-cached string result.

It seems to be Time.parse rather than :localtime:

Warming up --------------------------------------
          Time.parse     1.439k i/100ms
           localtime     1.427k i/100ms
    localtime parsed   481.587k i/100ms
    Utils.parse_date   201.939k i/100ms
Calculating -------------------------------------
          Time.parse     14.465k (± 1.7%) i/s -     73.389k in   5.075361s
           localtime     14.221k (± 2.0%) i/s -     71.350k in   5.019481s
    localtime parsed      4.798M (± 1.3%) i/s -     24.079M in   5.019862s
    Utils.parse_date      1.982M (± 1.6%) i/s -     10.097M in   5.096938s

@fauno I just came across an observation that calling :localtime on a Time object actually mutates the given Time object. Can you confirm..?

Sorry, I came back to see what happened with this PR and it seems I missed the email! I confirmed with documentation and a quick freeze test (Time.now.utc.freeze.localtime throws FrozenError) so I changed the PR a little. Should I rebase?

You need not rebase just yet, @fauno unless you'd like to use this branch in one of your projects.

Ok, everything seems fine, thanks for reviewing :)

@jekyll: merge +minor