Chrono

======

When including the additional header file :file:`pybind11/chrono.h` conversions

from C++11 chrono datatypes to python datetime objects are automatically enabled.

This header also enables conversions of python floats (often from sources such

as ``time.monotonic()``, ``time.perf_counter()`` and ``time.process_time()``)

into durations.

An overview of clocks in C++11

------------------------------

A point of confusion when using these conversions is the differences between

clocks provided in C++11. There are three clock types defined by the C++11

standard and users can define their own if needed. Each of these clocks have

different properties and when converting to and from python will give different

results.

The first clock defined by the standard is ``std::chrono::system_clock``. This

clock measures the current date and time. However, this clock changes with to

updates to the operating system time. For example, if your time is synchronised

with a time server this clock will change. This makes this clock a poor choice

for timing purposes but good for measuring the wall time.

The second clock defined in the standard is ``std::chrono::steady_clock``.

This clock ticks at a steady rate and is never adjusted. This makes it excellent

for timing purposes, however the value in this clock does not correspond to the

current date and time. Often this clock will be the amount of time your system

has been on, although it does not have to be. This clock will never be the same

clock as the system clock as the system clock can change but steady clocks

cannot.

The third clock defined in the standard is ``std::chrono::high_resolution_clock``.

This clock is the clock that has the highest resolution out of the clocks in the

system. It is normally a typedef to either the system clock or the steady clock

but can be its own independent clock. This is important as when using these

conversions as the types you get in python for this clock might be different

depending on the system.

If it is a typedef of the system clock, python will get datetime objects, but if

it is a different clock they will be timedelta objects.

Provided conversions

--------------------

.. rubric:: C++ to Python

- ``std::chrono::system_clock::time_point`` → ``datetime.datetime``

    System clock times are converted to python datetime instances. They are

    in the local timezone, but do not have any timezone information attached

    to them (they are naive datetime objects).

- ``std::chrono::duration`` → ``datetime.timedelta``

    Durations are converted to timedeltas, any precision in the duration

    greater than microseconds is lost by rounding towards zero.

- ``std::chrono::[other_clocks]::time_point`` → ``datetime.timedelta``

    Any clock time that is not the system clock is converted to a time delta.

    This timedelta measures the time from the clocks epoch to now.

.. rubric:: Python to C++

- ``datetime.datetime`` or ``datetime.date`` or ``datetime.time`` → ``std::chrono::system_clock::time_point``

    Date/time objects are converted into system clock timepoints. Any

    timezone information is ignored and the type is treated as a naive

    object.

- ``datetime.timedelta`` → ``std::chrono::duration``

    Time delta are converted into durations with microsecond precision.

- ``datetime.timedelta`` → ``std::chrono::[other_clocks]::time_point``

    Time deltas that are converted into clock timepoints are treated as

    the amount of time from the start of the clocks epoch.

- ``float`` → ``std::chrono::duration``

    Floats that are passed to C++ as durations be interpreted as a number of

    seconds. These will be converted to the duration using ``duration_cast``

    from the float.

- ``float`` → ``std::chrono::[other_clocks]::time_point``

    Floats that are passed to C++ as time points will be interpreted as the

    number of seconds from the start of the clocks epoch.