recommended way to floor time_point to a given duration

Question

I have a user-provided duration

const int period_seconds = cfg.period_seconds;

I would like to floor a time_point to the granularity of that duration.

I see that there is a std::chrono::floor function for time_point and duration, but it's not immediately obvious how to use them to do what I'm looking for.

I have the following toy example which works:

const auto seconds = std::chrono::seconds{period_seconds};
const auto period = std::chrono::duration_cast<std::chrono::nanoseconds>(seconds);
const auto now = std::chrono::system_clock::now();
const auto floored = now - (now.time_since_epoch() % period);

The net effect of the above code, for a duration of 10 seconds would be the following:

now:     2022-04-19 15:06:26.781772408
floored: 2022-04-19 15:06:20.000000000    # so "floors" the 25.78 seconds to 20 seconds

I can't help but feel that working in terms of the time_since_epoch and a modulus operator isn't the "right" way of getting the result I'm looking for...

Is there a "better" / recommended way to floor a time_point to some user-provided duration?

Answer 1

Your solution looks pretty good to me. However it can be slightly simplified. There's no need to form the nanoseconds-precision period. You can operate directly with seconds:

const auto floored = now - (now.time_since_epoch() % seconds);