As many know, a Martian day, or Sol, is 39 minutes and 35 seconds longer than a Terrestrial day. This 2.7% difference can wear down a team on Mars-time both mentally and physically. The solution for the MER teams was to have a self-winding mechanical watch altered to run slightly slower by Garo Anserlian, master watch and clockmaker.
Not having $300 to spend when the MER's first landed and not being able to justify it now, Ara Kourchians open-sourced the software to make an electronic one based on a Ti Chronos ez430. The clock, which has built in RF communication, timers, alarms, accelerometer, barometer, and a temperature sensor, was reprogrammed to be accurate over the next few decades (or as accurate as the original clock would have been).
For those with smartphones, there are Mars clocks for BlackBerry ($0.99), iPhone (free), and Android (free). This is how the majority of Curiosity's team keeps Mars time. In an IAmA on Reddit, surface systems engineer Eric Blood said that "Some of us do [have wristwatches calibrated to Mars Time], but a lot of us have iPhone and Android apps with Mars time."