Why we get differences between RTCs?

According to the article in wikipedia.org:

Timing

Most RTCs use a crystal oscillator,^[4][5] but some use the power line frequency.^[6] In many cases the oscillator's frequency is 32.768 kHz.^[4] This is the same frequency used in quartz clocks and watches, and for the same reasons, namely that the frequency is exactly 2¹⁵ cycles per second, which is a convenient rate to use with simple binary counter circuits.

We can assume that the differences could be materials or methods that leads to different oscillator's frequencies. In time those differences could be reflected in seconds, minutes... I've seen clocks with up to 7 minutes of difference. If you are in a AD environment and have some linux computers authenticating against a DC those differences could lead you to trouble (Mostly if you use SAMBA shares). A command like the following used in a crontab could fix the issue really simple:

# crontab -l

0 23 * * * /sbin/ntpdate name-of-the-dc-server

Windows 7: Allowing Access to C$

After recently installing Windows 7, I found that I was unable to access the "default admin shares" for my local drives, namely \\ME\C$. I was able to see the machine on my local network, so I know that the firewall wasn't the problem. If I shared out something explicitly, I could see that from other machines as well: File Sharing was turned on. On other networks, I knowthat I've been able to access the C$ share from other machines, so the functionality was still there, but likely just turned off.
From HowToGeek.com, I found a registry setting that would allow this. Go to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System, add a new DWORD called LocalAccountTokenFilterPolicy and set its value to 1 and reboot. Want to revert? Delete the key or set it 0.