Normative: Correct Gregorian calendar BC(E) era to signed years less than 1 AD

Fixes #723

Author: gibson042

spec/datetimeformat.html

@@ -1197,7 +1197,7 @@ <h1>
           </tr>
           <tr>
             <td>[[Era]]</td>
-            <td>Let _year_ be YearFromTime(𝔽(floor(_tz_ / 10<sup>6</sup>))). If _year_ &lt; *-0*<sub>𝔽</sub>, return *"BC"*, else return *"AD"*.</td>
+            <td>Let _year_ be YearFromTime(𝔽(floor(_tz_ / 10<sup>6</sup>))). If _year_ &lt; *1*<sub>𝔽</sub>, return *"BC"*, else return *"AD"*.</td>
           </tr>
           <tr>
             <td>[[Year]]</td>