As long as you test in advance of your planned travel date you can activate your pass with a date in the future and add travel days etc.

You then cancel those travel plans and deactivate your pass until the day you travel.

You can cancel travel days up to 2359 CET the previous day, but after that you will lose that day if you then do not travel. Hence the regular advice not to activate the pass until you know your first train is happening. The same advice for future travel days, do not start a travel day by transferring a journey from your pass until the day you know you are travelling. 

Here is some useful information from the experienced travellers in the Community regarding both planning, reservations and activation of pass and travel days. 

 Planning

The rail planner is normally not up to date, as it only is updated once a month, so to be sure of the time table you better check the timetable and availability on the websites of the national railways. The bigger national railways, like DB (Germany) SBB (Switzerland) and ÖBB (Austria) cover several countries. 

 Reservations 

Thanks Yorkie.

Just to confirm i thinking of purchasing the ticket tomorrow and my buying the 7 days in 1 month pass and my travel dates are between 5/10 -21/10. Are you suggesting that I add a trip to test for some day after 21/10 ?

Not necessay to go that far.

Simply load your pass as per instructions and create your trip. You do not need to activate your pass at this stage.  

Then add a journey to your trip for a date a few days in the future.

Now activate your pass for that date. It will create a travel dayl for that date.

Now you add the journey to your pass by moving the slider.

If all is well you should now open your pass and see if it shows the journey.

If it shows all is well.

Back to your trip and deactivate the slider.

Then delete the travel day and deactivate your pass.

This must be done at least one full day efore your planned start date

Then only use the planner and trip functions until you start travelling.

The video on this page might be useful as it explains pass activation and set up step by step. 

I just noticed below on the website re activation. Do I need to go to a train station in person to activate this if I dont eurorail to activate when ordering given I am not 100% sure the days I am travelling ? Is there another options

You can get your pass activated at a European train station, or when you place your order at Eurail.com/ Interrail.eu. This service is always free of charge.

1. Activate your Eurail Pass for free at a European train station.
To do this, simply go to the ticket office of any large train station in one of the participating Eurail countries. A railway official will then activate your Eurail Pass by filling in the start and end date of your Pass, as well as your passport number. They will also stamp the Pass in order to validate it.

2. Get your Eurail/ Interrail Pass activated for free when you place your order at Eurail.com/ Interrail.com.
We can activate your Eurail/ Interrail Pass before you receive it. If you use this service your Eurail/ Interrail Pass will arrive pre-activated. This option is useful if you already know the exact dates you're going to travel. Simply select the 'activate my pass' option, and pick the starting date of your trip.

You can only activate your pass online at the time of booking. Please note that we'll ask you to fill in your passport number. If you're not yet in possession of the passport you'll be traveling to Europe with, you can't make use of pre-activation.

I believe this is relating to paper passes.

Mobile passes are activated and deactivated as detailed above  by you at your leisure, 

As before please trial to your own satisfaction but always several days ahead of any trial trains you add. You cannot deactivate a current travel day once created.