If you want to create filters for dates in the future, you need to use negative numbers. For example, if you want to send an auto message to customers 3 days before their subscription ends, you would create a "subscription_ends_at" custom attribute and filter for "more than -3 days ago" and "less than 0 days ago." This will match all users whose subscription is about to end in 3 days time.
To ensure this works, the custom attribute must be sent to Intercom as a Unix Timestamp and the type must be set to 'Date'.
Custom attributes ending with "_at" are automatically treated as a date, rather than a number. However, if you named your custom attribute differently, you can change the type to 'Date' manually in your app settings under 'People Attributes'.
If you're using the 'exactly x days ago' filter we'll find users who match this filter during a 24 hour period. For example, 'signed_up_at exactly -3 days' ago will find users whose signed_up_at date is 2-3 days from now (and not 3-4 days from now).