March 17 appears to be the correct date.
In the Chinese Wikipedia article on the Xuzhou campaign, the timeline has him dying on the 17th.
His Baidu biography goes into detail about the action on each day of the Battle of Teng County. It describes the Japanese breaching the walls on the 3rd day (16th), before describing Wang's death the following day when he was directing battle in the town centre's crossroads and was struck by multiple bullets.
In 2003, his birthplace of Xindu erected a bronze statue in memory of Wang Mingzhang. They date they chose was March 17.