Our customers are trying to set up their pass for tomorrow (21st December) but when adding the pass to the app it seems to think the trip starts today (20th December) and wants to use an additional pass day when it should not. We have asked them to uninstall and reinstall the app and the error is still happening.
Which timezone are they in? If they're not already in Europe, it's probably better to wait until they are. This may be another issue related to timezone differences, which can pop up when the user is in another timezone than CET (UTC+1) and the date is different in both timezones.
There's no need to activate the pass in advance. In fact, it's better not to do that because something unexpected might happen and you can only cancel the activation until midnight CET before validity starts. It's totally fine to do the activation a few minutes before boarding the first train.
You can test that activation works by selecting s start date well in the future and then cancelling the activation immediately afterwards.
Which timezone do your customers use? If they could set the timezone to Central European Time (GMT+1) on their device, it might solve the issue
If not, best use customer support in this case, make sure you mention the date so that your question gets priority. Not a lot of us can help with technical difficulties related to the app on this community: https://eurail.zendesk.com/hc/en-001/requests/new
Thanks - they’re in the UK so GMT, never had this issue before! Tried to contact Interrail support but kept getting directed here 
Will reach out to them on the link provided - appreciate the quick responses!
The advice is to wait with the activation until just before boarding the first train of the travel. Please read more below.
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
The advice from the experienced travellers in the community is to use other ways to make reservations than the Interrail/Eurail website. You can look at the guide in the link:
https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105
If you, after having looked at the guide, have questions about how to make specific reservation, please give your travel details (departure date, time and route) preferably in a new topic, and you will get advice.
Please note that Interrail/Eurail charges an extra fee of 2 EUR per person and train in addition to the fee for the seat reservation.
Activation of pass (mobile pass)
During the activation process, when you choose the start day of the validity of the pass, the first day of the validity period is automatically made a travel day, even if you don't enter a journey, the advice is therefore not to activate the pass before the first travel day as you only can deactivate the pass before 00.00 on the day the validity starts. If your travel plans change in the last moment you will loose travel days if you have activated the pass in advance.
It can be wise to make a test and activate the pass with a start date well in the future and then deactivate the pass immediately, just to see that everything works.
Activation of travel day (mobile pass)
The advice from the experienced travellers in the community is also never to activate a travel day, that is connect a journey to your pass and create the ticket (QR code), until just before boarding the train, otherwise you might loose a travel day if your travel plans change in a late stage You can't delete a travel day in the past. A travel day can only be deleted until 23.59 CET the day before the travel day.
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