TEA has a good calendar (actually, it is a GTK+ 2 widget). But TEA provide some additional functionality. With Days between section you can calculate how many days are between twi selected dates. Select the first date in the calendar and press the date1 button. Then select another date and press the date2 button. Press Calculate button to see the result at Logmeno.
Below the calendar widget you see another buttons beginning from Insert.... With them you can insert a date into the current document in different formats.
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