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are you using the datejs framework??
I had also Dates where I got a "RangeError"! In my case it was my fault, because I inculded unintended the datejs library twice! (It is also a problem if you include core.js and date.js!!)
Just a suggestion....
No, I don't think ExtJS or I use Datejs.
But, anyway, thank you for the feedback.
Did you mean this Datejs?