How to calculate period between a specific date? - flutter

Question

I'm studying Flutter and in my application I need to calculate how many days are left until the 17th of the next month. There is no way to have a specific date, so it would be crucial to have today's date and add 1 month and calculate how many days are left.
For example: Today is 2021-09-09 and there are 38 days to 2021-10-17. The function does not calculate the 17th of the current month, but the following month.
Any idea how to make a function that takes the current date, adds 1 month and calculates how many days are left until the 17th? Thanks.

Answer 1

Basically copy the DateTime object with 1 added to the current month number and the day set to the 17th. Then subtract the two DateTime objects and convert the resulting Duration to days. It's important to perform all calculations in UTC so that Daylight Saving Time adjustments aren't a factor.
/// Returns the number of days to the specified [day] in the month following
/// [startDate].
int daysToNextMonthDay(DateTime startDate, int day) {
// Recreate `startDate` as a UTC `DateTime`. We don't care about the
// time.
//
// Note that this isn't the same as `startDate.toUtc()`. We want a UTC
// `DateTime` with specific values, not necessarily the UTC time
// corresponding to same moment as `startDate`.
startDate = DateTime.utc(startDate.year, startDate.month, startDate.day);
var nextMonthDay = DateTime.utc(startDate.year, startDate.month + 1, day);
// Verify that the requested day exists in the month.
if (nextMonthDay.day != day) {
throw ArgumentError(
'Day $day does not exist for the month following ${startDate.month}',
);
}
return nextMonthDay.difference(startDate).inDays;
}
void main() {
var now = DateTime(2021, 9, 9);
print(daysToNextMonthDay(now, 17)); // Prints: 38
}

Answer 2

try this:
void main() {
DateTime date = DateTime.now();
print(daysLeft(date.subtract(Duration(days: 1))));
}
bool isLeapYear(int year) {
if (year % 4 == 0) {
if (year % 100 == 0) {
if (year % 400 == 0) {
return true;
} else {
return false;
}
} else {
return false;
}
} else {
return false;
}
}
int daysLeft(DateTime date){
int daysLeft = 0;
switch(date.month){
case 1:
case 3:
case 5:
case 7:
case 8:
case 10:
case 12: daysLeft = 31 - date.day + 17; break;
case 4:
case 6:
case 9:
case 11: daysLeft = 30 - date.day + 17; break;
case 2: if(isLeapYear(date.year)){
daysLeft = 29 - date.day + 17;
break;
}else{
daysLeft = 28 - date.day + 17;
}
}
return daysLeft;
}