Well, this topic seems pretty simple, but many people forget that we can have the SQL Engine, Tools and other components like Reporting Services, Analysis Services in different versions or installed on different dates, so the expiration date may change accordingly. For e.g., you may have installed the Engine first and then decided to install Management Tools later on.
If you are using an Evaluation edition of SQL Server 2005/2008 then you must already know that Evaluation Edition works for 180 days from the date of installation. So, it’s very important for you to know:-
1. When was your SQL Server Instance Installed
2. When it is going to stop working. Once the 180 days are up, the service will not start.
Read on if you don’t want to see this message once the trial period is up 
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So I thought I’d post the various different way a DBA can go about determining the install date for SQL Server Evaluation Edition, thus calculating when the 180 day trial period expires. Here are some ways to go about finding this information:-
1. Run the following query on the evaluation instance (assuming the 180 days are still NOT over and SQL service is still running),
SELECT create_date as ‘SQL Server Install Date’,
DATEADD(dd,180,create_date) as ‘Instance will Stop Working ON’
FROM sys.server_principals WHERE name=‘NT AUTHORITY\SYSTEM’
-> ‘NT AUTHORITY\SYSTEM’ is SQL Server principal which gets created during the installation. So we are just checking the creation date of that security principal.
2. Look at the setup logs located in "%ProgramFiles%\Microsoft SQL Server\100\Setup Bootstrap\Log\". Sort by Date Modified ascending and you should get the install date. This method doesn’t work if the log directory was cleaned up or moved or deleted.
3. Open the Management Studio on the SQL Server, click on the “Help” of Menu Bar and then on “About”. In the new window you will see some thing like this:
Microsoft SQL Server Management Studio (expires in 104 days)
Note: This is assuming that you installed SSMS along with Engine as part of the same original install at the same time.
4. You can check the windows installer for the SQL Product code to pull out the install date. In registry look under,
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\UserData\S-1-5-18\Products\<GUID>\InstallProperties
The "InstallDate" key will have the date stored in YYYYMMDD format.
For SQL 2008 you can search the registry for "Microsoft SQL Server 2008 Database Engine Services" to get the correct GUID.
For SQL 2005 you can search the registry for "Microsoft SQL Server 2005" only to get the correct GUID.
Note: Keep in mind the instance name if you have more than 1 instance installed on the same machine
While on this topic, keep in mind there is an issue when you perform an in-place edition upgrade of an evaluation edition to a licensed edition. You will still get the error for SSMS or any tools. This is a known issue. Please follow the following KB to fix this issue.
The SQL Dude! 