I frequently need a tool to calculate how many days from a date to another, for example from 27 April 2016 to 18 July 2016.
Are there tools for Windows or for OS X?
On OS-X you already have python installed and on windows it is easily installed so you can simply do it on the command line, from your example:
>python -c"import datetime as dt;d=dt.date(2016,7,18) - dt.date(2016,4,27);print d.days" 82
You can even get time involved, e.g.:
>python -c"import datetime as dt;d=dt.datetime(2016,7,18,16,30) - dt.datetime(2016,4,27,20,43);print d" 81 days, 19:47:00
If you are using python 3 you need to make the prints functions. Obviously you can put things into python scripts for more power and flexibility.
See the online manuals for more details.
http://www.timeanddate.com/date/duration.html has a nice UI, friendly and useful but it requires internet connectivity..