This site provides an online "Days From Date" calculator to help you find the date that occurs exactly X days from a particular date. You can also enter a negative number to find out when X days before that date happened to fall. You can use this tool to figure out a deadline if you have a certain number of days remaining. Or read the full page to learn more about the due date if you're counting business days or weekdays only, skipping Saturday and Sunday.
Note: If you're trying to measure the number of days between two dates, you can switch to the Date Difference calculator instead. Or if you want to choose a day in the week and count how many times it occurs in a given year, try out the How Many Days calculator.
Today is Tuesday, December 11th, 2018. It is the 345th day of the year, and in the 50th week of the year (assuming each week starts on a Monday). There are 31 days in this month. 2018 is not a leap year, so there are 365 days in this year.