std::chrono::nonexistent_local_time in C++

The C++20 standard includes the header, which defines the std::chrono::nonexistent_local_time exception. It describes an error state in which a local time cannot be converted to a corresponding std::chrono::sys_time because the time is "nonexistent", which frequently occurs during daylight saving time (DST) transitions.

The std::chrono::nonexistent_local_time exception is issued when an attempt is made to convert a nonexistent std::chrono::local_time to std::chrono::sys_time without indicating how to handle the ambiguous circumstance. The conversion could occur in operations such as:

1. Std::chrono::time_zone::to_sys: This function converts the local time to the system time (sys_time). Suppose the local time occurs inside the skipped period during a DST transition, and no specific handling strategy (such as selecting the earliest or latest possible acceptable time) is specified. In that case, this function will throw a std::chrono::nonexistent_local_time exception.
2. Constructors of std::chrono::zoned_time: When creating a std::chrono::zoned_time object from a std::chrono::local_time or if the provided local time is invalid due to DST modifications, the constructor will throw this exception unless a resolution strategy is specified.

Syntax:

It has the following syntax:

Parameters:

• tp: This parameter (std::chrono::local_time) represents the current local time.
• Duration: A template parameter provides the time unit (seconds or milliseconds).
• i: This is a std::chrono::local_info object that contains additional information about the time zone and DST rules, which helps to contextualize the nonexistent time.

Understanding the Non-existent Local Time

Nonexistent local time refers to times that do not exist in a specific time zone due to adjustments, such as daylight saving time transitions. For example, when clocks are set forward during DST, some local times are skipped. It means that precise times, such as 2:30 AM on the first day of daylight saving time, do not exist because the clock jumps from 2:00 AM to 3:00 AM.

Example:

Let us take an example to illustrate the std::chrono::nonexistent_local_time function in C++.

Output:

 
The Local time is: 2024-03-18  04:50:00    

Explanation:

This C++ program demonstrates how to deal with possible issues when converting a std::chrono::system_clock::time_point to a local time representation. In the print_local_time function, the time_point is first converted to std::time_t, which indicates the time in seconds since the epoch. After that, the std::time_t value is converted to a std::tm structure using std::localtime, resulting in a human-readable local time format. If std::localtime returns nullptr that indicates an issue caused by an incorrect time (perhaps due to daylight saving time transitions), an exception is thrown and caught. The error message is shown with std::cerr. If the conversion succeeds, the local time is formatted and output with std::put_time. To show how the process operates, the main function creates a time_point representing a given epoch time and executes print_local_time.

Use Cases:

Several use cases of the std::chrono::nonexistent_local_time function in C++ are as follows:

1. Scheduling Applications:

Applications that allow users to schedule events, such as calendar apps or task managers, must handle times that fall into nonexistent periods due to DST transitions.

2. Time-Based Transactions:

Financial systems and transaction tracking depend on precise timestamps must account for nonexistent times to keep correct records.

3. Time-sensitive Notifications:

Applications that send reminders or notifications based on local time need to account for changes due to DST.

4. Automated Systems:

Automated systems, such as smart home devices, that perform actions based on local time must handle cases where certain local times are skipped due to DST.

5. Time-sensitive notifications:

Applications that deliver reminders or notifications depending on local time must accommodate changes due to DST.

6. Data logging and analysis:

Systems that log data at regular intervals must handle periods when timestamps may be skipped or duplicated due to DST.

7. Travel and booking systems:

To ensure precise booking systems, travel booking systems that deal with various time zones must accurately handle DST transitions into and out of.

8. User Interfaces:

Applications with user interfaces displaying local time must handle DST transitions smoothly to avoid showing nonexistent or incorrect times.