You can use the mkpasswd tool to do this. There's a good primer on how to use it over on cyberciti.biz, titled: Linux / UNIX: Generating random password with mkpasswd.
Example
mkpasswd --char=10 --crypt-md5
The package is usually called makepasswd, but the tool is typically called mkpasswd.
See the man page for more details.
Generating contents of /etc/shadow directly
The following python command will generate the portion that goes into the /etc/shadow file:
Python
$ python -c "import crypt, getpass, pwd; \
print crypt.crypt('password', '\$6\$SALTsalt\$')"
Perl
$ perl -e 'print crypt("password","\$6\$saltsalt\$") . "\n"'
Which generates the following output:
$6$SALTsalt$UiZikbV3VeeBPsg8./Q5DAfq9aj7CVZMDU6ffBiBLgUEpxv7LMXKbcZ9JSZnYDrZQftdG319XkbLVMvWcF/Vr/
Changing the /etc/shadow
Here's a command that will change the existing entry with the newly generated password field from the command above.
perl -pe 's|(root):(\$.*?:)|\1:\$6\$SALTsalt\$UiZikbV3VeeBPsg8./Q5DAfq9aj7CVZMDU6ffBiBLgUEpxv7LMXKbcZ9JSZnYDrZQftdG319XkbLVMvWcF/Vr/:|' /etc/shadow > /etc/shadow.new
NOTE: This is a rough example but works. You'll need to take the results from the command that generated the hashed password, and escape the dollar signs ($) with slashes (\$).
The results are stored in a new file /etc/shadow.new. Once you've confirmed the results you can replace /etc/shadow with the new file, /etc/shadow.new.
Best Answer
The "seconds since 1970" timestamp is specifically defined as UTC in most usages. In particular, you may notice that
date +%sgives the same result asdate -u +%s.The relevant line where this is set in the shadow password utilities is"
Which would make it UTC. SCALE is defined as 86400 (except via a specific ifdef that I can't quite trace what circumstances cause to be defined)