findMinDate method
Implementation
DateTime? findMinDate(int year, int month) {
DateTime? start;
_nodeMap.forEach((key, value) {
if (key.year == year && key.month == month) {
if (start == null) {
start = key;
} else {
if (key.isBeforeDay(start!)) {
start = key;
}
}
}
});
return start;
}