Rollover to 2010 has caused an "invalid date" error.

Happy New Year to All,

Please see below what has happened to my Archive Calendar, and the error below that.

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January 2010

Mon Tue Wed Thu Fri Sat Sun
-3 -2 -1 0 1 2 3
4 5 6 7 8 9 10
11 12 13 14 15 16 17
18 19 20 21 22 23 24
25 26 27 28 29 30 31

warning: cal_days_in_month() [function.cal-days-in-month]: invalid date. in \........\sites\all\modules\archive\archive.module on line 106.

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Thanks
Stephen