I have a calculated item ( Numeric Unsigned, unit s ) which does the following:

last(system.localtime) - last(puppet.run.timestamp)

It should return me the number of seconds since the last puppet run. Which it does fine until a new puppetrun runs.
These are the values from the item:

2012.Nov.05 10:01:52 157
2012.Nov.05 09:59:52 157
2012.Nov.05 09:57:52 157
2012.Nov.05 09:55:52 157
2012.Nov.05 09:53:52 18446744073709551173
2012.Nov.05 09:51:52 3158
2012.Nov.05 09:49:52 3158
2012.Nov.05 09:47:52 3158
2012.Nov.05 09:45:52 3158
2012.Nov.05 09:43:52 2557
2012.Nov.05 09:41:52 2557
2012.Nov.05 09:39:52 2557
2012.Nov.05 09:37:52 2557
2012.Nov.05 09:35:52 2557
2012.Nov.05 09:33:52 1957
2012.Nov.05 09:31:52 1957
2012.Nov.05 09:29:52 1957
2012.Nov.05 09:27:52 1957
2012.Nov.05 09:25:52 1957
2012.Nov.05 09:23:52 1357
2012.Nov.05 09:21:52 1357
2012.Nov.05 09:19:52 1357
2012.Nov.05 09:17:52 1357
2012.Nov.05 09:15:52 1357
2012.Nov.05 09:13:52 757
2012.Nov.05 09:11:52 757

My guess is, because system.localtime is older ( and thus smaller, runs every 15 mins ) than puppet.run.timestamp ( updated every 120 secs ), the calculation is negative, and instead of returning 0, it overflows to a giant number?