dateTimeLexer top-level property
Follows DateTime format specified in FHIRPath (I have also updated the FHIR) package to follow many of these guidelines
Implementation
final dateTimeLexer = (char('@') &
dateFormatLexer &
char('T') &
(timeFormatLexer & timeZoneOffsetFormatLexer.optional()).optional())
.flatten()
.map((value) => DateTimeParser(value));