The configuration
Calendars, week boundaries, time zones and holiday tables — where a one-country system breaks.
One System, Several Calendars
A time system configured for the first country keeps producing confident answers in the second, and nothing about the output indicates which values it used.
Where the Week Begins
A weekly limit needs a week, and which day it starts on is a local matter that most systems hold as a single global setting.
Time Zones and the Shift at Midnight
A system holding one time zone will mis-date shifts in every other, and the errors concentrate in night work, which is where the limits are strictest.
Daylight Saving, Twice a Year
Two nights a year produce a shift that is an hour short and one that is an hour long, and countries change their clocks on different dates or not at all.
The Holiday Table
The most visibly local piece of configuration, maintained in most organisations by one person from a web page every December.
Rounding and Local Expectations
A rounding rule that is unremarkable in one country can be unlawful in another, and it is usually a single global setting nobody has examined.
Testing a Second Country First
A rollout that adds a country by copying the previous one inherits every assumption. Half a day of deliberate testing catches almost all of it.