lastday function
Implementation
DateTime lastday(DateTime d) {
int lastDay = getMonthDays(d.year, d.month);
String dayLast =
"${d.year.toString().padLeft(4, "0")}/${d.month.toString().padLeft(2, "0")}/${lastDay.toString().padLeft(2, "0")}";
return strToDate(dayLast);
}