isAfter function
Returns true if str represents a date after reference.
When reference is omitted the current UTC time is used.
Implementation
bool isAfter(String str, [String? reference]) {
final date = tryParseDate(str);
if (date == null) return false;
final ref =
reference == null ? DateTime.now().toUtc() : tryParseDate(reference);
if (ref == null) return false;
return date.isAfter(ref);
}