Travel Day versus Travel Month
A rail day is considered any travel between midnight of one day to midnight of the following day, in other words, a 24 hour period. During this period, you may take as many train trips as you like and it will only count as one day. A good tip to know is the 7pm rule: if you board a direct, overnight train after 7pm, only the following day of travel is counted. On your pass, you would write the day of arrival on the pass prior to boarding the overnight train. Some individual country passes do not offer this rule.
A rail month is calculated as a calendar month. For example, if your pass begins on January 2, it is valid until midnight on February 1.