<p>Yes.  Use the <code>format='fits'</code> to the time function.</p>
<blockquote>
<p>FITS format: “[±Y]YYYY-MM-DD[THH:MM:SS[.sss]]”.</p>
<p>ISOT but can give signed five-digit year (mostly for negative years);</p>
</blockquote>
<p>(<a href="https://docs.astropy.org/en/stable/time/index.html#:%7E:text=2000.45-,fits,%E2%80%982000%2D01%2D01T00%3A00%3A00.000%E2%80%99,-gps" rel="nofollow noreferrer">documentation</a>)</p>

CC BY-SA 4.0
2023-04-22T06:20:57.990Z
2023-04-22T06:20:57.990Z
James K
<p>The goal is to find precessed coordinates for BCE dates.</p>
<pre><code>from astropy.time import Time
equinox = Time('-0500-01-01') 
</code></pre>
<p>is just not working, though '0500-01-01' works just fine.  Is there a time format that allows BCE dates or years before present or anything like that?</p>

CC BY-SA 4.0
2023-04-22T02:20:32.380Z
What does the documentation say?
planetmaker
CC BY-SA 4.0
2023-04-22T06:13:17.370Z
The documenation https://docs.astropy.org/en/stable/time/index.html  It says "iso 8601 compliant date" but iso just says implementations may use an expanded date format, by prior agreement.
James K
<astropy><precession>
Astropy time: is it possible to use negative dates?
J Kelly
# Astropy time: is it possible to use negative dates?

(Asked by: username_0 on 2023-04-22)

The goal is to find precessed coordinates for BCE dates.
from astropy.time import Time
equinox = Time('-0500-01-01') 

is just not working, though '0500-01-01' works just fine.  Is there a time format that allows BCE dates or years before present or anything like that?

**Tags:** astropy, precession
> **Comments:**
> **username_1**: What does the documentation say?  
> **username_2**: The documenation https://docs.astropy.org/en/stable/time/index.html  It says "iso 8601 compliant date" but iso just says implementations may use an expanded date format, by prior agreement.  

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## 1 answers

### Answer by username_2 on 2023-04-22. Score: 3 | Accepted answer

Yes. Use the `format='fits'` to the time function.

> FITS format: “[±Y]YYYY-MM-DD[THH:MM:SS[.sss]]”.
>
> ISOT but can give signed five-digit year (mostly for negative years);

([documentation](https://docs.astropy.org/en/stable/time/index.html#:%7E:text=2000.45-,fits,%E2%80%982000%2D01%2D01T00%3A00%3A00.000%E2%80%99,-gps))


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