Calculate number of days based on criteria

I need to calculate occupancy time for a population of migratory animals. Occupancy is defined as the period, in days, between the first and last sighting of an individual in a given year. A small example of the type of sighting data I am dealing with in Excel:

I need to be able to calculate the number of days between sightings within a year and not between the first and last sightings or between each sighting. I'm wondering if there is a way to do this with DATEDIF and IF statements.