Integer noOfDays = startDate.daysBetween(endDate);
The above formula is used to find number of days between two dates.
Execute the below code in Developer Console to view the output:
Date startDate = Date.today();
Date endDate = Date.today().addDays( 90 );
Integer noOfDays = startDate.daysBetween( endDate );
system.debug( 'Output is ' + noOfDays );
Date endDate = Date.today().addDays( 90 );
Integer noOfDays = startDate.daysBetween( endDate );
system.debug( 'Output is ' + noOfDays );