Use Program RS_ABAP_SOURCE_SCAN , and provide inputs as mentioned below [ String searched for 'Field name 'and in Package 'Z*']you will get all the places where you have used this particular field across system.
Use FM " ISU_NUMBER_DAYS_OF_RANGE ", provide inputs as mentioned in below attachment. Based on Number of days in output 365/366 .. you can differentiate between Leap year or not.or You can use another FM HR_99S_INTERVAL_BETWEEN_DATES
