Try not and "overthink" this to much. If the locomotive has a fire in the fire box and steam on the boiler anytime during the calendar day, that is a single service day. If you get past midnight, and there still is a fire with steam pressure, that is 2 service days and so on.
If you fire a boiler up at say, 9:00 am, gain steam by 1pm, and then immediately kill it at 1:05 pm, that is still a service day. If you fire it up again the next day (with steam) that is another service day.
Bottom line is, anytime a boiler has a fire in it, with pressure, at anytime during the calendar day (midnight to 11:59 am), that is a service day. It doesn't make any difference if the locomotive never moves or if it is under steam for only a few minutes or the entire day....it counts as a service day.
Hope this helps.....