Resolving SQLite strftime Format Discrepancies and Feature Requests for AM/PM, 12-Hour Clocks, and ISO Week Numbers

Inconsistent AM/PM Formatting and Missing 12-Hour Clock/ISO Week Support in SQLite’s strftime Function

Issue Overview: Conflicting Implementations of %p/%P Modifiers and Absence of Standardized 12-Hour Time/ISO Week Formats

The core issue revolves around three interrelated problems with SQLite’s implementation of the strftime function:

1. Case Sensitivity Inconsistencies with %p and %P Modifiers
   The %p format specifier returns uppercase "AM"/"PM" in SQLite, while %P returns lowercase "am"/"pm". This conflicts with expectations from other implementations like Python’s strftime (which only supports %p for uppercase) and Linux’s C library (where %P is a GNU extension for lowercase). Developers expect case consistency across platforms but face divergent behavior when migrating between systems.

2. Lack of Native 12-Hour Clock Formatting Without String Manipulation
   SQLite historically required convoluted expressions like CASE WHEN CAST(strftime('%H', datetime_column) AS INTEGER) >= 12 THEN 'PM' ELSE 'AM' END to derive AM/PM indicators. While version 3.44 introduced %I (12-hour hour with leading zero) and %p/%P, there’s no direct equivalent to format "12" instead of "00" for midnight/noon in 12-hour notation. The original request sought %h to represent "12" when the hour modulo 12 equals 0.

3. Missing ISO 8601 Week Numbering (%V) and Year-of-Week Formats
   ISO week numbers (%V) differ from calendar weeks, as weeks start on Monday and the first week must contain at least four January days. Calculating this requires non-trivial logic absent in SQLite’s strftime, forcing developers to implement custom solutions. The lack of %G (ISO year) compounds this issue when dealing with cross-year weeks.

These problems stem from SQLite’s design philosophy of closely mirroring the C standard library’s strftime while balancing feature requests. The tension between backward compatibility, cross-platform consistency, and developer convenience creates friction when handling localized time formatting and international standards.

Possible Causes: Historical Adherence to C strftime Standards vs Modern Formatting Needs

The root causes of these discrepancies lie in SQLite’s architectural decisions and evolving requirements:

1. C Standard Library Compatibility as Primary Reference
   SQLite’s strftime was initially designed to mirror the C89 standard’s behavior, which defines %p for locale-dependent AM/PM in uppercase. The GNU extension %P (lowercase) was later adopted but remains non-standard. This explains why %P behaves differently across platforms – SQLite follows the underlying C library’s implementation where it exists, leading to platform-specific inconsistencies.

2. Deliberate Exclusion of Non-Standard Format Specifiers
   Format specifiers like %h (12-hour with 00→12) and %V (ISO week) are not part of ISO C standards. SQLite maintainers have historically avoided adding non-standard extensions unless overwhelmingly requested, to prevent fragmentation. This conservatism forces developers to use workarounds like:

   SELECT (strftime('%I', 'now') % 12) || CASE WHEN (strftime('%H', 'now') >= 12) THEN ' PM' ELSE ' AM' END;
   

   Such workarounds are error-prone and impact readability.

3. Case Sensitivity in Format Specifiers
   The decision to make %P output lowercase in SQLite (unlike Python’s lack of support for %P) stems from interpreting lowercase as the "modified" version of %p. However, this violates the principle of least surprise, as developers expect case-insensitive modifiers or consistent casing across similar specifiers.

4. Complexity of ISO Week Calculation
   Implementing %V and %G requires adhering to ISO 8601 rules, which involve:

   • Weeks starting on Monday
   • The first week containing at least four days of the new year
   • Year transitions where December 31 might belong to week 1 of the next year
     Adding this to SQLite would necessitate significant code changes and testing across edge cases, a deterrent for inclusion without clear demand.

Troubleshooting Steps, Solutions & Fixes: Achieving AM/PM Consistency, 12-Hour Clock Adjustments, and ISO Week Workarounds

Resolving AM/PM Case Sensitivity and 12-Hour Formatting Issues

1. Standardizing AM/PM Case Output
   To enforce consistent casing regardless of %p or %P:

   -- Force uppercase using %p
   SELECT strftime('%I:%M %p', '2024-05-09 14:30:00') AS time_upper; -- 02:30 PM
   
   -- Force lowercase using lower(%p)
   SELECT lower(strftime('%I:%M %p', '2024-05-09 14:30:00')) AS time_lower; -- 02:30 pm
   
   -- Use %P directly if lowercase is acceptable
   SELECT strftime('%I:%M %P', '2024-05-09 14:30:00') AS time_lower_direct; -- 02:30 pm
   

   Note: %P is available starting SQLite 3.44. For older versions, combine %p with lower().

2. Converting 00 to 12 in 12-Hour Time
   SQLite lacks a built-in modifier to display "12" instead of "00" for midnight/noon. Use arithmetic and string replacement:

   SELECT 
     CASE 
       WHEN CAST(strftime('%I', datetime_column) AS INTEGER) = 0 THEN '12'
       ELSE strftime('%I', datetime_column)
     END || strftime(':%M %p', datetime_column) AS time_12h;
   

   For SQLite 3.44+ with %I support:

   SELECT 
     REPLACE(
       strftime('%I:%M %p', datetime_column), 
       '00:', 
       '12:'
     ) AS time_12h_fixed;
   

3. Leveraging New strftime Features in SQLite 3.44+
   Version 3.44 introduced several format specifiers:

   • %I: 12-hour hour with leading zero (00-12)
   • %l: 12-hour hour without leading zero (0-12)
   • %p/%P: AM/PM in uppercase/lowercase
     Example:

   SELECT strftime('%l:%M %P', '2024-05-09 00:30:00') AS time_12h; -- 12:30 am
   

Implementing ISO Week Number and Year Calculations

1. Custom ISO Week Number Calculation
   Use a combination of existing modifiers and arithmetic:

   WITH dates AS (
     SELECT '2023-01-01' AS date
     UNION ALL SELECT '2023-12-31'
     UNION ALL SELECT '2024-01-01'
   )
   SELECT 
     date,
     CASE 
       WHEN strftime('%w', date) = '0' THEN 7  -- Convert Sunday (0) to 7
       ELSE CAST(strftime('%w', date) AS INTEGER)
     END AS iso_weekday,
     (strftime('%j', date) + 7 - iso_weekday) / 7 AS iso_week
   FROM dates;
   

   This approximates ISO weeks but may fail near year boundaries.

2. Handling ISO Year Boundaries
   For accurate ISO year (%G), determine if the week belongs to the previous/next year:

   SELECT 
     date,
     CASE 
       WHEN iso_week = 1 AND strftime('%m', date) = '12' THEN strftime('%Y', date) + 1
       WHEN iso_week >= 52 AND strftime('%m', date) = '01' THEN strftime('%Y', date) - 1
       ELSE strftime('%Y', date)
     END AS iso_year
   FROM (
     -- Subquery with iso_week calculated as above
   );
   

3. User-Defined Functions for ISO Support
   For complex scenarios, extend SQLite using loadable extensions:

   #include <sqlite3ext.h>
   SQLITE_EXTENSION_INIT1
   
   static void isoWeekNumber(sqlite3_context *context, int argc, sqlite3_value **argv) {
       // Implement ISO week logic in C
   }
   
   int sqlite3_extension_init(sqlite3 *db, char **pzErrMsg, const sqlite3_api_routines *pApi) {
       SQLITE_EXTENSION_INIT2(pApi);
       sqlite3_create_function(db, "iso_week", 1, SQLITE_UTF8, NULL, isoWeekNumber, NULL, NULL);
       return SQLITE_OK;
   }
   

   Compile as a loadable extension and use:

   SELECT iso_week('2024-12-31') AS week_num; -- Returns 1 (ISO week of 2025)
   

Ensuring Cross-Platform Compatibility with strftime Implementations

1. Testing for %p/%P Case Behavior
   Verify SQLite’s behavior with:

   SELECT 
     strftime('%p', '2024-05-09 09:00:00') AS upper_am,
     strftime('%P', '2024-05-09 21:00:00') AS lower_pm;
   -- Returns 'AM' and 'pm' in SQLite 3.44+
   

   Adjust application code to normalize case if necessary.

2. Adopting strftime Wrapper Functions
   Create a wrapper function to harmonize output:

   CREATE VIEW formatted_time AS
   SELECT 
     datetime_column,
     REPLACE(
       strftime('%I:%M %p', datetime_column), 
       '00:', 
       '12:'
     ) AS time_12h,
     lower(strftime('%p', datetime_column)) AS am_pm_lower
   FROM events;
   

3. Version-Specific Feature Detection
   Check for %I/%P support using pragma:

   SELECT sqlite_version() AS version;
   -- If version >= 3.44, use native modifiers
   

   Fallback to legacy methods if older versions are detected.

By systematically addressing each discrepancy through SQL-native functions, user-defined extensions, and careful testing, developers can mitigate the limitations of SQLite’s strftime while advocating for future inclusion of ISO week and enhanced time formatting features.