C++ Datetime library

Introduction

ggdatetime is a C++ library to assist date and time related computations in Geodesy related fields.

Compilation / Installation

Source code is ISO C++17. Compilation should be trivial using any C++ compiler supporting the c++17 standard (option -std=c++17 in gcc and clang).

October 2021 Source code can also be compiled using the latest C++ 20 standard.

Since December 2021, the build system has been changed from GNU Autotools to scons. Since September 2024 the build system has been changed to cmake

Compile and install

Supposing you are located in the top-level directory:

## to build in a folder named "build":
$> cmake -S . -B build -DCMAKE_BUILD_TYPE=Release
$> cmake --build build --target all --config Release -- -j4
## Install, system-wide (needs root)
$> make install
## (Optional) run tests
$> ctest --test-dir build

Examples

Consult the test programs source code in the test folder for examples on how to use the library.

Developers

Take a look at the LLVM Coding Standards and if possible stick to it.

Use clang-format to format you code before commiting; if you try to commit with non-acceptable code format, the CI system will fail.

The Library

Nomenclature

• Julian year 365.25 days
• Julian century 36525 days

Julian date (JD) is a continuous count of days (JD zero is located about 7000 years ago). Because of the unwieldy size of Julian Dates and the awkwardness of the half-day offset, it is accepted practice to remove the leading ‘24’ and the trailing ‘.5’, producing what is called the Modified Julian Date, i.e. $ MJD = JD − 2,400,000.5 $

In the library, this constant is named ds::MJD0_JD = 2400000.5e0, i.e. the Julian Date of Modified Julian Date zero.

Julian epoch is a convinient way of representing an epoch as fractional years. It uses the Julian year of exactly 365.25 days, and the TT time scale; Julian epoch 2000.0 is defined to be 2000 January 1.5 (i.e. 2000 January 1d 12h), which is JD 2,451,545.0 (i.e. dso::J2000_JD) or MJD 51,544.5 (i.e. dso::J2000_MJD). Julian epoch is usually denoted as “J2000.0”.

Precision

Transforming from datetime<T> to TwoPartDates

Transforming from a datetime<nanoseconds> instance to an instance of type TwoPartDate preserves a precision better than 2e-2 nanoseconds. This is verified by running the test program unit_tests/tpdates1.cpp

Transforming from a datetime<microseconds> instance to an instance of type TwoPartDate preserves a precision better than 2e-5 microseconds. This is verified by running the test program unit_tests/tpdates2.cpp

Transforming from a datetime<milliseconds> instance to an instance of type TwoPartDate preserves a precision better than 2e-8 milliseconds. This is verified by running the test program unit_tests/tpdates3.cpp

Transforming from a datetime<seconds> instance to an instance of type TwoPartDate preserves a precision better than 2e-12 seconds. This is verified by running the test program unit_tests/tpdates4.cpp

Transforming from/to Julian Epoch

Transforming from a TwoPartDate to a Julian Epoch and back, preserves an accuracy of ~1e-5 seconds. This is verified by running the test program test/sofa/epj_date.cpp.

Authors

Dionysos Satellite Observatory, School of Rural Surveying and Geoinformatics Engineering, National Technical University of Athens

Send bugs to: Xanthos Papanikolaou, xanthos@mail.ntua.gr