A day is classified as one calendar day – not 24 hours from the start of the journey. BritRail Passes are valid from 00H00 to 24h00 (midnight to midnight) on a chosen day of travel.
You need to commence your last journey of the day before midnight. If you are travelling on an overnight sleeper train, the next day’s date must be inserted (i.e. sleeper trains count as TWO days travel, not one).