What is the valid range for DateTime? #146
Description
I am planning to prepare a pull request that expands the documentation of the DateTime class, which I find somewhat terse and incomplete as it stands now. I would like to add a clear statement about the range of dates that the class can correctly handle. It seems quite clear to me that the earliest valid date is 2000-01-01, but what is the other end of the range?
Looking at the source code, there seem to be various dates at which different parts of the implementation break:
| date | breakage |
|---|---|
| 2068-01-19 03:14:08 | return type of secondstime() |
| 2100-01-01 00:00:00 | comment for date2days() |
| 2100-03-01 00:00:00 | computation of leap years |
| 2106-02-07 06:28:16 | return type of unixtime() |
| 2136-02-07 06:28:16 | secondstime(), if made unsigned |
| 2179-06-07 00:00:00 | return type of date2days() |
| 2256-01-01 00:00:00 | data type of yOff |
- The first limit is probably an oversight that can be easily fixed by returning
uint32_tinstead oflong. This would raise the limit ofsecondstime()to 2136-02-07. - The second is just a comment, though I understand the value of specifying a limit that has no technical justification but is easy to remember.
- The computation of leap years can be easily fixed, but that would add a small cost in code size and execution time. It may not be worth it.
- The
unixtime()limit would be harder to fix without resorting to 64-bit arithmetics. One option would be to specify that the method returns only the lowest 32 bits of the whole result, and let the user figure out the other bits if she needs them, following the approach of NTPv3. Calculations that useunixtime()internally would switch to usingsecondstime(). - Beating the 2136 limit would probably be way too costly.
I am leaning towards specifying the limit as either 2100-03-01 (that date excluded, to avoid the cost of a better leap year calculation) or 2100-01-01 (nice round number easy to remember). But since I am in no position to decide, I need feedback from the maintainers.