With the JR Pass does the first day start at midnight or at the time of your first rail trip eg. 7.00 pm?
Exact validity period
The Japan Rail Pass effectively starts at 12:00:01AM on your first day of use, and ends at midnight (11:59:59) on the last day of use. This is the JR standard rules whether you first board a train at 6AM or at 6PM on your first day.
Key Point: the validity is counted in rounded days, not hours.
You are free to travel any time on the starting date/day of your Japan Rail Pass. it will expire on midnight of your last day, after this you cannot board a train, but if you are already on a train you can continue to your destination at no additional cost. (eg. you can use it on board a sleeper train for example, that arrives after the last day of validity, as long as you depart within the valid period)
There's more on the jrpass.com blog: https://jrpass.com/blogs/can-i-use-my-jr-pass-with-breaks-between-days

