CDST - A DST libary in C

This repository provides a DST library for C-based projects which require Daylight Saving Time functionality.

The library is based on current DST transitions as described by Wikipedia. One exception is the Lord Howe Island in Australia.

As DST is a binary state, the library features one function that checks if the current time, expressed as 32-bit unsigned UNIX time, is within the specified country's DST window. The window is stored as part of the library and is referenced by the country's ISO 3166-1 numeric code.

Note: The library doesn't take into account local variations inside a country, like Hawaii which doesn't have DST. The primary target for this library is embedded devices which don't have access to external clock sources, such as online tz-databases.

To save space, the library can be compiled only for specific continents. This can be done by changing the continent's option in the CMakeLists.txt or on the command-line with "-DDISABLE_ASIA_RULES=ON". By default, all DST rules are compiled.

Note: If a continent's rules are disabled, the library will always return "false", i.e. "DST is not currently active".

Requirements

• GCC
• CMake
• Ruby (testing only)

Testing

For verification the library uses Unity and CTest to run a set of unit tests. The unit tests can be compiled by changing the BUILD_UNITTEST option to ON.

Test timestamp generation

Test timestamps for each country's transitions was generated using the following Python sequence:

import pytz, datetime

tz = "Europe/Berlin"
num_entries = 30

for dt in pytz.timezone(tz)._utc_transition_times[-num_entries]:
    print(dt, dt.replace(tzinfo=datetime.timezone.utc).timestamp())