Problem about positive time error with Xbee

Hello there! I'm trying to use RTC and Xbee in my project. I put the receive node which is Xbee router with PC and monitored by XCTU software. But the time seems have some problem, because the time I got will increase slowly! I thought it's RTC's problem ,but there's same situation without RTC just send the data every 4s. Here's the log I took from XCTU.

02-17-2017 10:37:31.999,19,RECV,7E0020900013A20040D8DA685C220152534C4F570000000000000000000000000000004A
02-17-2017 10:37:35.989,20,RECV,7E0020900013A20040D8DA685C220152534C4F570000000000000000000000000000004A
02-17-2017 10:37:39.999,21,RECV,7E0020900013A20040D8DA685C220152534C4F570000000000000000000000000000004A
02-17-2017 10:37:44.009,22,RECV,7E0020900013A20040D8DA685C220152534C4F570000000000000000000000000000004A
02-17-2017 10:37:48.009,23,RECV,7E0020900013A20040D8DA685C220152534C4F570000000000000000000000000000004A
02-17-2017 10:37:52.011,24,RECV,7E0020900013A20040D8DA685C220152534C4F570000000000000000000000000000004A
02-17-2017 10:37:56.011,25,RECV,7E0020900013A20040D8DA685C220152534C4F570000000000000000000000000000004A
02-17-2017 10:38:00.021,26,RECV,7E0020900013A20040D8DA685C220152534C4F570000000000000000000000000000004A
02-17-2017 10:38:04.011,27,RECV,7E0020900013A20040D8DA685C220152534C4F570000000000000000000000000000004A
02-17-2017 10:38:08.021,28,RECV,7E0020900013A20040D8DA685C220152534C4F570000000000000000000000000000004A
02-17-2017 10:38:12.021,29,RECV,7E0020900013A20040D8DA685C220152534C4F570000000000000000000000000000004A
02-17-2017 10:38:16.031,30,RECV,7E0020900013A20040D8DA685C220152534C4F570000000000000000000000000000004A
02-17-2017 10:38:20.031,31,RECV,7E0020900013A20040D8DA685C220152534C4F570000000000000000000000000000004A
02-17-2017 10:38:24.031,32,RECV,7E0020900013A20040D8DA685C220152534C4F570000000000000000000000000000004A
02-17-2017 10:38:28.041,33,RECV,7E0020900013A20040D8DA685C220152534C4F570000000000000000000000000000004A
02-17-2017 10:38:32.031,34,RECV,7E0020900013A20040D8DA685C220152534C4F570000000000000000000000000000004A
02-17-2017 10:38:36.041,35,RECV,7E0020900013A20040D8DA685C220152534C4F570000000000000000000000000000004A

As you can see the time received are increasing make the time not that accurate. Anyone have same problem here?