Why can the same date mean zero or one day?
These are two mathematical conventions. Date subtraction measures the difference between dates. Inclusive counting also counts the starting day. Neither convention tells you which tariff a carrier applies.
From 10 to 10 September, the date difference is 0 days and inclusive counting gives 1 day. From 10 to 13 September, the difference is 3 days; counting both dates gives 4: 10, 11, 12 and 13 September.
Count the period, then subtract free time
Assume calendar days, both dates included, from 10 to 17 September. This gives 8 days. With 5 free days, 3 are chargeable. Using the date difference instead gives a 7-day period and 2 chargeable days.
Chargeable days should not be negative. In this simplified calculation, a period shorter than the allowance produces zero charges, not a credit.
Split the days into rate bands
Assume 5 chargeable days: the first 2 at EUR 30 and the next 3 at EUR 50. One container costs 2 × 30 + 3 × 50 = EUR 210. Two containers with identical dates and conditions would cost EUR 420.
Check whether band numbering refers to all days in the period or only chargeable days. “From day 6” may need a different input from “the first day after free time”.
Record the counting rule with the result
Before sending the calculation, check the dates, inclusion of the starting day, free time, bands and currency. Identify the tariff source in your notes so somebody else can reproduce the result.
These examples use calendar days. If the contract excludes weekends or holidays, or applies specific cut-off times, those conditions need separate treatment. Choosing a date-subtraction convention does not account for them.