isWithinLast function
Returns true if str is in the past and no older than duration.
A date in the future returns false.
isWithinLast(signupDate, const Duration(days: 30));
Implementation
bool isWithinLast(String str, Duration duration) {
final date = tryParseDate(str);
if (date == null) return false;
final now = DateTime.now().toUtc();
return !date.isAfter(now) && date.isAfter(now.subtract(duration));
}