Issue #16285 has been reported by alanwu (Alan Wu).

----------------------------------------
Bug #16285: Setting mtime to zero Zlib::GzipWriter does nothing
https://bugs.ruby-lang.org/issues/16285

* Author: alanwu (Alan Wu)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN
----------------------------------------
Run the following script to observe the issue
```ruby
require 'zlib'

def write_gzip_file(content, mtime)
  File.open('archieve.gz', 'w') do |f|
    gz = Zlib::GzipWriter.new(f)
    gz.mtime = mtime
    gz.write(content)
    gz.close
  end
  system("file archieve.gz")
end

write_gzip_file("current time", 0)
```

Output: 
```
ruby 2.6.5p114 (2019-10-01 revision 67812) [x86_64-darwin18]
archieve.gz: gzip compressed data, last modified: Wed Oct 30 18:04:07 2019, from Unix, original size 12
archieve.gz: gzip compressed data, last modified: Thu Jan  1 00:00:01 1970, from Unix, original size 21
```

As you can see, setting mtime to 0 doesn't set the timestamp in the file header to be all zeros.
The documentation for `#mtime=` says:

> If you do not set an mtime, the default value will be the time when compression started. Setting a value of 0 indicates no time stamp is available.

which sounds like setting 0 should be a valid. 

I have a PR to fix this: https://github.com/ruby/zlib/pull/10



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