# Chrono

When including the additional header 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

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.

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.