Add support for working with negative dates

See discussion https://groups.google.com/g/tiddlywiki/c/aHlyaHr93Io/m/vGcDa6lxAgAJ

editions/tw5.com/tiddlers/features/DateFormat.tid

@@ -1,48 +0,0 @@
-created: 20140418142957325
-modified: 20180730201626985
-tags: Features
-title: DateFormat
-type: text/vnd.tiddlywiki
-
-When used to display date values (with the `format` attribute set to ''date''), the ViewWidget accepts a `template` attribute that allows the format of the date values to be specified. The format string is processed with the following substitutions:
-
-|!Token |!Substituted Value |
-|`DDD` |Day of week in full (eg, "Monday") |
-|`ddd` |Short day of week (eg, "Mon") |
-|`DD` |Day of month |
-|`0DD` |Adds a leading zero |
-|`DDth` |Adds a suffix |
-|`WW` |ISO-8601 week number of year |
-|`0WW` |Adds a leading zero |
-|`MMM` |Month in full (eg, "July") |
-|`mmm` |Short month (eg, "Jul") |
-|`MM` |Month number |
-|`0MM` |Adds leading zero |
-|`YYYY` |Full year |
-|`YY` |Two digit year |
-|`wYYYY` |Full year with respect to week number |
-|`wYY` |Two digit year with respect to week number |
-|`hh` |Hours |
-|`0hh` |Adds a leading zero |
-|`hh12` |Hours in 12 hour clock |
-|`0hh12` |Hours in 12 hour clock with leading zero |
-|`mm` |Minutes |
-|`0mm` |Minutes with leading zero |
-|`ss` |Seconds |
-|`0ss` |Seconds with leading zero |
-|`XXX` |Milliseconds |
-|`0XXX` |Milliseconds with leading zero |
-|`am` or `pm` |Lower case AM/PM indicator |
-|`AM` or `PM` |Upper case AM/PM indicator |
-|`TZD` |Timezone offset |
-|`\x` |Used to escape a character that would otherwise have special meaning |
-|`[UTC]`|Time-shift the represented date to UTC. Must be at very start of format string|
-
-Note that other text is passed through unchanged, allowing commas, colons or other separators to be used.
-
-! Examples
-
-|!Template |!Output |
-|`DDth MMM YYYY` |16th February 2011 |
-|`DDth MMM \M\M\M YYYY` |16th February MMM 2011 |
-|`DDth mmm YYYY 0hh:0mm:0ss` |16th Feb 2011 11:38:42 |