The daysBetween() method can be used to find number of days between two dates.

Example:

Integer noOfDays = startDate.daysBetween( endDate );

Execute the following sample code in Developer Console to view the output:

Sample Code:

Date startDate = Date.today();
Date endDate = Date.today().addDays( 90 );
Integer noOfDays = startDate.daysBetween( endDate );
system.debug( 'Output is ' + noOfDays );