Unable to create a property in a RFXCOM Unit

[Hendrik van Eeden]

I have a RFXCOM temperature unit that monitors my freezer temperature.  I want it to send me a SMS when the temperature goes above 6°F.  However the freezer goes into a defrost mode for about 45 minute at which time the temperature reaches about 36 °F, the temperature then returns to normal.  So, I only want an SMS when the temperature stays above 6 °F for longer than 45 minutes.
I attempted to use a unit property to help me with the logic, but I can not create one.
I am on a mac mini running version 10.8.5 and XTension version 7 build 847. 
It doesn't even have a "lasteditunit" property.
Am I missing something?

Henk



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