> Ouch, is Python crashes because of an unsupported strftime call?

It's not a crash. It's a CRT error report dialog, which is enabled by default for the _CRT_ASSERT and _CRT_ERROR macros in debug builds. This dialog can be helpful when debugging interactively. It gives a developer the option to abort, retry, or ignore. If a debugger is attached, the retry option breaks into the debugger. I attached a debugger to the test runner to diagnose the problem with format code "%4Y". For example:

    >>> time.strftime('%4Y')
    Debug Assertion Failed!
    [...]
    (Press Retry to debug the application)
    (1a6c.101c): Break instruction exception - code 80000003 (first chance)
    ucrtbased!_Wcsftime_l+0x5af:
    00007ff8`a582b9af cc              int     3

backtrace:

    0:000> kc 4
    Call Site
    ucrtbased!_Wcsftime_l
    ucrtbased!_Strftime_l
    ucrtbased!strftime
    python311_d!time_strftime

locals:

    0:000> dv
          _Expr_val = 0n0
             string = 0x00000226`fe38d1c0 ""
           max_size = 0x400
             format = 0x00000226`fe37e7d0 "%4Y"
            timeptr = 0x00000041`c1feeda0
        lc_time_arg = 0x00000000`00000000
             locale = 0x00000000`00000000
      locale_update = class _LocaleUpdate
          format_it = 0x00000226`fe37e7d2 "4Y"
          remaining = 0x400
            lc_time = 0x00007ff8`a5868550
             failed = true
          string_it = 0x00000226`fe38d1c0 ""

resume, with the STATUS_BREAKPOINT (0x80000003) exception handled:

    0:000> gh
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    ValueError: Invalid format string
    >>> 


For non-interactive testing the report mode needs to be configured to 0 (no report) or to report to stderr. For example:

    >>> msvcrt.CrtSetReportMode(msvcrt.CRT_ERROR, msvcrt.CRTDBG_MODE_FILE)
    4
    >>> msvcrt.CrtSetReportFile(msvcrt.CRT_ERROR, msvcrt.CRTDBG_FILE_STDERR)
    18446744073709551615
    >>> os.abort()
    abort() has been called

For time.strftime('%4Y'), the source of the report is _VALIDATE_RETURN(false, EINVAL, 0) in _Wcsftime_l(). This macro includes an _ASSERT_EXPR(expr, msg) check. In a debug build, this calls _CrtDbgReportW(_CRT_ASSERT, ...) if the expression is false. If the latter returns 1 (retry), the __debugbreak() intrinsic is called to break into the debugger. To suppress the dialog, either temporarily set the CRT_ASSERT report mode to 0, or set it to report to stderr. For example:

    >>> msvcrt.CrtSetReportMode(msvcrt.CRT_ASSERT, msvcrt.CRTDBG_MODE_FILE)
    4
    >>> msvcrt.CrtSetReportFile(msvcrt.CRT_ASSERT, msvcrt.CRTDBG_FILE_STDERR)
    18446744073709551615

    >>> time.strftime('%4Y')
    minkernel\crts\ucrt\src\appcrt\time\wcsftime.cpp(1163) : Assertion failed: false
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    ValueError: Invalid format string