Based on your example, for an invoice issued on the 1st of the month to be collected on the 15th of the month, set it to collect 14 days after issue.
The number of days is after the issue date, with day 0 being the date of issue. For example, if I wanted to collect it on the 7th of the month (1 week after issue), I would set it to 6 days (1st + 6 days = 7th).
Bear in mind that if that date falls on a bank holiday or weekend, then it will be the next possible date.