MacBook Pro suddenly shutdown , date and time reset to Jan 1.

MacBook Pro suddenly shutdown , date and time reset to Jan 1.

Hi. My MacBook Pro (MD212) sometimes shutdown (not "gracefully") with no warning given. The date and time reset to Jan. 1 midnight. This happens when the notebook is on battery power. To be specific, it's more likely to happen when the remain battery is between 30%-60%. I have never experienced this issue when plugged in. And the sudden shutdown happens under both Yosemite and El Capitan.