Many news outlets are running pictures taken by the Mars rover Opportunity today, on the tenth anniversary of its launch. Pictures taken by Opportunity have a date stamp that may seem surprising: for example, sol 3353. Time keeping on Mars is an interesting detail of the mission.
A day on Mars is 2.4% longer than a day on Earth. Mission control crews work on Martian time. A day-shift assignment will turn into a night shift assignment, “sliding approximately 40 minutes later in Earth time each day. Wristwatches calibrated in Martian time, rather than Earth time, are used by many of the team members.” NASA scientists use the convention that the first day of operations of a Lander is called Sol 0, so I suppose pictures taken on the same day by different landers will have different Martian date stamps. There aren’t enough landers for that to get confusing… yet.
The motions of a planet make working by its own day and year the obvious choice. For craft in space, I suppose there is no reason to abandon Earth time, and specifically the Earth time of its mission control base. For anniversary celebrations, we might as well use Earth time. After all, we are the one’s who are celebrating.