DCode is a portable tool which will convert various binary date/time formats into the actual date which they represent.
Why bother? Imagine you're browsing the Registry for some reason. You find a potentially useful key, like HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Windows\ShutdownTime, which tells you when the system was last shut down - but only in some unintelligible string of hex digits, like "f8 5f 80 4d 05 77 cc 01". Normally at this point you'd probably give up and move on, but DCode can take this kind of value and actually convert it into something meaningful.
And it's easy to use, too. Just enter or copy data into the "Value to Decode" box, click Decode and read the time it represents (Monday, 19 September 2011 19:49:54 UTC, in this case).
Note that the default format for something like this - "Windows: 64 bit Hex value - Little Endian" - will be fine for many Registry system times. There are many other binary formats you might encounter, though, and DCode supports most of them: Windows 64 bit (big endian), Windows Cookie, Windows 64 bit OLE, Windows SYSTEM Structure, Hotmail / Email Filetime, Unix 32 bit (little endian), Unix 32 bit (big endian), Unix numeric, Google Chrome, Apple Mac Absolute, MS-DOS wFatTime, wFatDate, MS-DOS wFatDate, wFatTime, HFS 32 bit (little endian), HFS 32 bit (big endian), HFS+ (big endian) and HFS+ (little endian). Just choose the appropriate format from a list.
DCode is one of those programs that you'll rarely ever need, but when you do, can be very useful. If you ever browse the Registry for any reason then it's worth the download.