Q:

How does duration_cast round

If I convert to a coarser unit of time (say std::chrono::minutes to std::chrono::hours) how will duration_cast round? For example, what value will std::chrono::minutes(91) become if converted to std::chrono::hours? 2h, 1h?

A:

duration_cast always rounds towards zero.  I.e. positive values round down and negative values round up.
For other rounding options see:
http://howardhinnant.github.io/duration_io/chrono_util.html
floor, ceil, and round are currently in the draft C++ 1z (hopefully C++17) draft working paper.  In the meantime feel free to use the code at chrono_util.html, and please let me know if you have any problems with it.

C++ 17 update

std::chrono::floor
std::chrono::ceil
std::chrono::round

std::chrono::floor<std::chrono::seconds>(1400ms)  ==  1s
std::chrono::floor<std::chrono::seconds>(1500ms)  ==  1s
std::chrono::floor<std::chrono::seconds>(1600ms)  ==  1s
std::chrono::floor<std::chrono::seconds>(-1400ms) == -2s
std::chrono::floor<std::chrono::seconds>(-1500ms) == -2s
std::chrono::floor<std::chrono::seconds>(-1600ms) == -2s

