Issue on Monitor plugin "16583d 14h 29m 6s"

[smlick]

I recently have had an issue in monitor plugin (cacti 0.8.8c), when a device go down I get ever a downtime of "16583d 14h 29m 6s".
If I look in device I get the appropriate downtime but under monitor interface I get ever "16583d 14h 29m 6s" and the counter is still in progress.
I tried server restart, I tried to disable, uninstall and reinstall the plugin.....but nothing.
....the counter is still in progress.
Why?
Some days ago was all ok.....

regards
Alessio