For many people, doing mental math with dates is difficult. This is true for a few reasons. First of all, doing mental math is hard in general. Secondly, and to make matters harder, date and time math is not done in base 10. What do I mean by that? For example: there are 60 seconds in a minute, there are 60 minutes in an hour, 24 hours in a day, 7 days in a week. It is easier for our brains to do math in increments of 10. For example, if I ask you what 100 * 10 is, you can probably answer instantly. If I ask you what 17 * 62 is, you will probably have a much more difficult time. To make matters worse, not all months have the same number of days (or weeks, technically). Months start on random days of the week and years end on random days of the week. Sometimes February has 29 days. The point that I'm trying to make is that mental math is hard, and that mental math with dates and times is even harder. That's why I made this tool.
It is much easier to have computers calculate these things for us. In this case, I've written a tool using the PHP programming language that allows you to enter your query into the URL and you'll get the answer to your question on the subsequent page. You can replace the parameters in the URL with your desired inputs. Acceptable "start" dates are "yesterday", "today", and "tomorrow". Acceptable units of times are "days", "weeks", "months", "years". You can see other example queries on this page.