如何使用R正确地在时间戳上添加毫秒

the unit of tick_count column is 100 millisecond. I want to plus the tick_count time on date_time. But the result doesn't execute addition correctly. For example, the second value should be "2014-08-15 12:51:02.6 EEST" rather than "2014-08-15 12:51:02.5 EEST", I don't know why this happened. It caused a serious impact on my result because some time it lead to two or three rows having a same time. How to deal with this situation?the unit of tick_count column is 100 millisecon