Changing Date in web query

How can I automatically change the date or enter a date in a cell to change where the web query gets the data table from? I also have a question on how you can do a date range? The dates are at the end of the url, and the ones with a range have the dates inside the url as well.

Some of the queries I want are for 14 days, 30 days, etc. 

Here are a couple examples. 

It seems some of the 14 day ones use actual dates and some may be based on the current date (9/26/17) in this example. 

Thanks in advance if you can help!

https://www.fangraphs.com/leaders.aspx?pos=all&stats=pit&lg=all&qual=0&type=8&season=2017&month=0&season1=2017&ind=0&team=0&rost=0&age=0&filter=&players=p2017-09-26

https://www.fangraphs.com/leaders.aspx?pos=all&stats=pit&lg=all&qual=0&type=8&season=2017&month=3&season1=2017&ind=0&team=0&rost=0&age=0&filter=&players=p2017-09-26

https://www.fangraphs.com/leaderssplits.aspx?splitArr=1&strgroup=season&statgroup=2&startDate=2018-2-13&endDate=2018-2-26&filter=&position=B&statType=team&autoPt=false&players=&sort=15,1&pg=0

 

Thanks in advance!