ErfaWarning: ERFA function "d2dtf" yielded 1 of "dubious year (Note 5)" when using `astropy.time.Time.to_datetime()`

I keep getting these warnings from the ERFA library when converting certain times to datetime objects:

/Users/lzkelley/anaconda3/lib/python3.5/site-packages/astropy/_erfa/core.py:118: ErfaWarning: ERFA function "d2dtf" yielded 1 of "dubious year (Note 5)"

No idea why, but I can reproduce it with:

> t = 1693612800.0
> ap.time.Time(t, format='unix').to_datetime()

File "/Users/lzkelley/anaconda3/bin/ipython", line 6, in <module>
    sys.exit(IPython.start_ipython())
  File "/Users/lzkelley/anaconda3/lib/python3.5/site-packages/IPython/__init__.py", line 119, in start_ipython
    return launch_new_instance(argv=argv, **kwargs)
  File "/Users/lzkelley/anaconda3/lib/python3.5/site-packages/traitlets/config/application.py", line 658, in launch_instance
    app.start()
  File "/Users/lzkelley/anaconda3/lib/python3.5/site-packages/IPython/terminal/ipapp.py", line 348, in start
    self.shell.mainloop()
  File "/Users/lzkelley/anaconda3/lib/python3.5/site-packages/IPython/terminal/interactiveshell.py", line 440, in mainloop
    self.interact()
  File "/Users/lzkelley/anaconda3/lib/python3.5/site-packages/IPython/terminal/interactiveshell.py", line 431, in interact
    self.run_cell(code, store_history=True)
  File "/Users/lzkelley/anaconda3/lib/python3.5/site-packages/IPython/core/interactiveshell.py", line 2717, in run_cell
    interactivity=interactivity, compiler=compiler, result=result)
  File "/Users/lzkelley/anaconda3/lib/python3.5/site-packages/IPython/core/interactiveshell.py", line 2827, in run_ast_nodes
    if self.run_code(code, result):
  File "/Users/lzkelley/anaconda3/lib/python3.5/site-packages/IPython/core/interactiveshell.py", line 2881, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-185-04e6f34e68d8>", line 1, in <module>
    ap.time.Time(t, format='unix').to_datetime()
  File "/Users/lzkelley/anaconda3/lib/python3.5/site-packages/astropy/time/core.py", line 1444, in to_datetime
    return tm._shaped_like_input(tm._time.to_value(timezone))
  File "/Users/lzkelley/anaconda3/lib/python3.5/site-packages/astropy/time/formats.py", line 596, in to_value
    self.jd1, self.jd2)
  File "/Users/lzkelley/anaconda3/lib/python3.5/site-packages/astropy/_erfa/core.py", line 21906, in d2dtf
    check_errwarn(c_retval_out, 'd2dtf')
  File "/Users/lzkelley/anaconda3/lib/python3.5/site-packages/astropy/_erfa/core.py", line 118, in check_errwarn
    warnings.warn('ERFA function "' + func_name + '" yielded ' + wmsg, ErfaWarning)
  File "/Users/lzkelley/Programs/zcode/zcode/inout/inout_core.py", line 845, in warn_with_traceback
    traceback.print_stack()
/Users/lzkelley/anaconda3/lib/python3.5/site-packages/astropy/_erfa/core.py:118: ErfaWarning: ERFA function "d2dtf" yielded 1 of "dubious year (Note 5)"
  warnings.warn('ERFA function "' + func_name + '" yielded ' + wmsg, ErfaWarning)
Out[185]: datetime.datetime(2023, 9, 2, 0, 0)

ap.version.githash: ‘552a2de83e23b89192a91b423eb69d35ef5424df’ ap.version.version: ‘1.3’

[pllim]

This has continued to confuse users downstream. I just got asked this question again today. @astrofrog said we should definitely make the warning less cryptic.

cc @bhilbert4

[lzkelley]

The note that the warning refers to is (from astropy/_erfa/core.py, which must be autogenerated somehow??):

    5) The warning status "dubious year" flags UTCs that predate the
       introduction of the time scale or that are too far in the future
       to be trusted.  See eraDat for further details.

what does “too far in the future to be trusted” mean? What sets the scale-time of trustability?