I always had the impression that Log4j `FixedDateFormat` is faster than Commons `FastDateFormat` which is faster than Java `DateTimeFormatter`, yet, unless I am doing something stupid, the picture is totally different in Java 17:
*Benchmark (pattern) Mode Cnt Score Error Units*commonsFdf HH:mm:ss.SSS thrpt 12 6749.822 ± 692.047 ops/s commonsFdf yyyy-MM-dd'T'HH:mm:ss.SSS thrpt 12 4377.318 ± 209.050 ops/s javaDtf HH:mm:ss.SSS thrpt 12 2515.950 ± 38.564 ops/s javaDtf yyyy-MM-dd'T'HH:mm:ss.SSS thrpt 12 1794.764 ± 51.975 ops/s log4jFdf HH:mm:ss.SSS thrpt 12 1032.089 ± 65.850 ops/s log4jFdf yyyy-MM-dd'T'HH:mm:ss.SSS thrpt 12 696.700 ± 40.708 ops/s The sources and results can be reached from this GitHub repository <https://github.com/vy/date-time-format-benchmark> and the associated GitHub Actions workflow runs <https://github.com/vy/date-time-format-benchmark/runs/4008433552>, respectively. The preliminary profiling points to `FixedDateTime#millisSinceMidnight(long)` for the unexpected slowness of `FixedDateTime`. Would anybody mind confirming that the measurements make sense and `FixedDateTime` is... broken?
