Well, nothing is fail-safe. There is always a way to break any kind of protection, even if it's iLock.
That said, here are the steps you should take.
1. Lock down the workbook and, more importantly, the VBA code. VB project protection is relatively difficult to break.
2. Make sure you have some kind of mechanism that checks whether or not macros are enabled. I'm obviously not familiar with your project, but many times my projects are mostly useable without macros. Have a "macros disabled" sheet, that is always activated on open, and hidden right away via code.
3. On first open, create a registry entry that records the date of first use. If such an entry already exists, check the date against today's date. Also, to be safe, record "last used date" and add 1 day to it on each use that the system date is before last used date. You could also have the expiration date hard-coded, but first use basis definitely looks more professional.
4. If trial period expires, prompt for registration key. Or, I would suggest instead having them request a full version - it looks a little less professional, but gives you a little more control.
Obvious workarounds are changing the system date changes (which I propose a solution for above), and going into the registry. Best bet is to have a registry key that is not obviously yours.
Think of this in terms of your customer. Is the customer's liason to you working on this project exclusively? If so, give them an exact date. Or, is the point of contact someone who hired you to make a spreadsheet because he/she doesn't have time to do it? Then, set a trial period as of first use.
You can track the user name and how long ago that user first used the program. Generally users will try something first, then e-mail it to their associates for a second/third/etc opinion. Let each user have 30 days; or, save the global usage time within the worksheet.
Those are just some ideas. Ideally, you'll think of every possible way to pirate your spreadsheet, and check for each one at each stage in the trial period. You can track the users, dates they've used it, number of usage times, etc. None of it is foolproof; be sure to test each use and misuse scenario that occurs. Look professional.
Hope that helps.
