why is std::chrono::duration based on seconds

Question

I'm learning <chrono> library, and considering the std::chrono::duration class, is there any specific reason to base it on seconds? For example a variable to store seconds would be

chrono::duration<int> two_seconds(2);

and all other time spans require relating them to seconds, like

chrono::duration<int, ratio<60>> two_minutes(2);
chrono::duration<int, ratio<1, 1000>> two_milliseconds(2);
chrono::duration<int, ratio<60 * 60 * 24>> two_days(2); 

Are there any reasons to base duration on seconds and not on minutes, hours, etc.?

Answer 1

Seconds are chosen because it's the basic time unit in both the SI system and in science as a whole.
Even Americans use seconds and not something like microfortnights.

why this basic time span isn't another template parameter for duration class

It is, as you can provide typedefs for ratios, and some are included in the standard.

#include <chrono>

std::chrono::duration<int, minutes> two_minutes(2);            // Standard
std::chrono::duration<int, milliseconds> two_milliseconds(2);  // Standard

If you need more, they're trivial to add:

typedef std::ratio<60 * 60 * 24> days;
typedef std::ratio<756, 625> microfortnights;

std::chrono::duration<int, days> two_days(2); 
std::chrono::duration<int, microfortnights> two_weeks(1000000);