https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126387
Bug ID: 126387
Summary: formatting of file_time time point wraps and can't
format whole representable time range
Product: gcc
Version: 17.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: hanicka at hanicka dot net
Target Milestone: ---
I have noticed that formatting file_time<nanoseconds> wraps unexpectedly:
This is the highest possible value I can print without wrapping:
```c+++
auto highest =
std::chrono::file_time<std::chrono::nanoseconds>{std::chrono::seconds{2785708036}
+ std::chrono::nanoseconds{854775807}};
std::cout << highest;
```
will print: "2262-04-11 23:47:16.854775807"
if I add one nanosecond:
```c++
auto wrapping =
std::chrono::file_time<std::chrono::nanoseconds>{std::chrono::seconds{2785708036}
+ std::chrono::nanoseconds{854775808}};
std::cout << wrapping;
```
will print: "1677-09-21 00:12:43.854775808"
notice the nanosecond part same, but dates and hh:mm:ss are different
completely.
your file_clock should represent range between years 1901-2446
https://github.com/gcc-mirror/gcc/blob/380bd5c411037ee59c1db96b6d0f882e06d91d5d/libstdc%2B%2B-v3/include/bits/chrono.h#L1491-L1494
When I found about the range, I was curious if someone Jonathan who picked the
range (or at least wrote the comment) is a trekkie, so I tried to find exact
date when the range ends, and found this bug.
In following compiler-explorer link I print multiple values:
min/zero/max/highest not wrapping/after wrap
https://compiler-explorer.com/z/zoevfzEMa
I would expect:
std::chrono::file_time<std::chrono::nanoseconds>{std::chrono::nanoseconds{std::numeric_limits<long
int>::min()}} should format to "1901-something"
std::chrono::file_time<std::chrono::nanoseconds>{std::chrono::nanoseconds{std::numeric_limits<long
int>::max()}} should format to "2446-something"
I guess similar problem will be (less pronounced) in formatting of other offset
timepoints to sys_clock.
