As part of RUST-1129, we discovered that a boundary condition on date representation was incorrect for relaxed extJSON but correct for canonical extJSON; this wasn't caught by the corpus tests because the [relevant test](https://github.com/mongodb/specifications/blob/372747fca128f449abbbb01b200dc9a2cabf522d/source/bson-corpus/tests/datetime.json#L24-L28) doesn't have relaxed expected output.

Potentially, in this case we could test against the canonical expected output (and recommend that for adoption in other drivers). However, initial test with that logic in place showed other failures that need to be investigated before that can move forward:

---- tests::spec::corpus::run stdout ----
{{ thread 'tests::spec::corpus::run' panicked at 'assertion failed: `(left == right)`: Multiple types within the same document: All BSON types}}Diff < left / right > :
{{ Object(}}{{{ "_id": Object( Unknown macro: }}
{{

{ "$oid"}}}

{{ ),}}
{{ "String": String(}}
{{ "string",}}
{{ ),}}
{{ < "Int32": Number(}}
{{ < 42,}}
{{ < ),}}
{{ < "Int64": Number(}}
{{ < 42,}}
{{ < ),}}
{{ < "Double": Number(}}
{{ < -1.0,}}
{{ < ),}}
{{ > "Int32": Object(}}
{{ Unknown macro: {> "$numberInt"}}}
{{ ),}}
{{ > "Int64": Object(}}
{{ Unknown macro: {> "$numberLong"}}}
{{ ),}}
{{ > "Double": Object(}}
{{ Unknown macro: {> "$numberDouble"}}}
{{ ),}}
{{ "Binary": Object(}}

{{ { "$binary": Object( Unknown macro: }}
{ "base64"}}}

{{ ),}}
{{ }),}}
{{ "BinaryUserDefined": Object(}}

{{ { "$binary": Object( Unknown macro: }}
{{{ "base64"}

{{ ),}}
{{ }),}}
{{ "Code": Object({}}
{{ "$code": String(}}
{{ "function() {}",}}
{{ ),}}
{{ }),}}
{{ "CodeWithScope": Object({}}
{{ "$code": String(}}
{{ "function() {}",}}
{{ ),}}
{{ "$scope": Object({}),}}
{{ }),}}
{{ "Subdocument": Object(}}
{{ Unknown macro: { "foo"}}}
{{ ),}}
{{ "Array": Array([}}
{{ < Number(}}
{{ < 1,}}
{{ < ),}}
{{ < Number(}}
{{ < 2,}}
{{ < ),}}
{{ < Number(}}
{{ < 3,}}
{{ < ),}}
{{ < Number(}}
{{ < 4,}}
{{ < ),}}
{{ < Number(}}
{{ < 5,}}
{{ < ),}}
{{ > Object(}}
{{ Unknown macro: {> "$numberInt"}}}
{{ ),}}
{{ > Object(}}
{{ Unknown macro: {> "$numberInt"}}}
{{ ),}}
{{ > Object(}}
{{ Unknown macro: {> "$numberInt"}}}
{{ ),}}
{{ > Object(}}
{{ Unknown macro: {> "$numberInt"}}}
{{ ),}}
{{ > Object(}}
{{ Unknown macro: {> "$numberInt"}}}
{{ ),}}
{{ ]),}}
{{ "Timestamp": Object(}}

{{ { "$timestamp": Object( Unknown macro: }}
{ "t"}

{{ ),}}
{{ }),}}
{{ "Regex": Object(}}

{{ { "$regularExpression": Object( Unknown macro: }}
{ "pattern"}

{{ ),}}
{{ }),}}
{{ "DatetimeEpoch": Object(}}

{{ { < "$date": String( < "1970-01-01T00:00:00Z", < ), > "$date": Object( Unknown macro: }}
{> "$numberLong"}}}

{{ ),}}
{{ }),}}
{{ "DatetimePositive": Object(}}

{{ { < "$date": String( < "1970-01-25T20:31:23.647Z", < ), > "$date": Object( Unknown macro: }}
{{{> "$numberLong"}

{{ ),}}
{{ }),}}
{{ "DatetimeNegative": Object(}}

{{ { "$date": Object( Unknown macro: }}
{ "$numberLong"}

{{ ),}}
{{ }),}}
{{ "True": Bool(}}
{{ true,}}
{{ ),}}
{{ "False": Bool(}}
{{ false,}}
{{ ),}}
{{ "DBRef": Object(}}

{{ { "$ref": String( "collection", ), "$id": Object( Unknown macro: }}
{{{ "$oid"}

}}),
{{ "$db": String(}}
{{ "database",}}
{{ ),}}
{{ }),}}
{{ "Minkey": Object(}}
{{ Unknown macro: { "$minKey"}}}
{{ ),}}
{{ "Maxkey": Object(}}
{{ Unknown macro: { "$maxKey"}}}
{{ ),}}
{{ "Null": Null,}}
{{ })}}