RB921GS-5HPacD wireless registration-table

Hello,
We installed some new mANTBox 15s devices. working nicely but not correct when reading the wireless clients registration table.
Are there more people experiencing this behaviour? And maybe found a solution?

I have upgraded to different versions of RouterOS but never solved my problem.
this is resulting in wrong graphs in my monitoring system.

[adonk@Zuidduinen-5Ghz-1] /system resource> print 
                   uptime: 7w5d7h4m47s
                  version: 6.35 (stable)
               build-time: Apr/14/2016 12:55:07
              free-memory: 97.0MiB
             total-memory: 128.0MiB
                      cpu: MIPS 74Kc V5.0
                cpu-count: 1
            cpu-frequency: 720MHz
                 cpu-load: 7%
           free-hdd-space: 111.2MiB
          total-hdd-space: 128.0MiB
  write-sect-since-reboot: 83920
         write-sect-total: 91406
               bad-blocks: 0%
        architecture-name: mipsbe
               board-name: RB921GS-5HPacD
                 platform: MikroTik


[adonk@Zuidduinen-5Ghz-1] /interface wireless registration-table> print count-only                                
601
[adonk@Zuidduinen-5Ghz-1] /interface wireless registration-table> print count-only where last-activity < "3600"      
55
[adonk@Zuidduinen-5Ghz-1] /interface wireless registration-table> print count-only where last-activity < "86400"    
124
[adonk@Zuidduinen-5Ghz-1] /interface wireless registration-table> print count-only where last-activity < "864000"
275
[adonk@Zuidduinen-5Ghz-1] /interface wireless registration-table> print count-only where last-activity < "8640000"
601

cheers
Arco

Last activity has a format like 00:00:00.120…
Can that comparison be done like that?