One month from today is not always 30 days
Learn why calendar-month addition differs from a fixed day count near February and month ends.
Choose a calendar rule first
Adding 30 days counts a fixed number of 24-hour date steps; adding one calendar month advances the month and then handles an invalid day such as the 31st. Different systems may clamp to the last valid day or roll into the following month. Specify the rule before using a date for a payment or deadline.
A month-end example
January 31 plus 30 days in a non-leap year is March 2. January 31 plus one calendar month can be February 28 under an end-of-month clamping rule. In a leap year the clamped date is February 29. The difference is not an arithmetic error: the operations ask different questions.
Check inclusive and business rules
A due date may count its starting day, exclude it, or move a weekend or holiday deadline. First decide whether the contract says days, calendar months or business days. Then calculate the exact date and verify the local rule for holidays and time zones. A calculator cannot interpret a contract for you.
Try it and keep reading
Try your numbers in date add, date difference. Then read the related guide for context and assumptions.