[chaley]

You don't say what a "bad date" looks like when showing in the booklist or book details. Because you are using a printable date (the value of '#column name') the month names will be localized. The date functions in calibre are localized to accept 3-letter month names in English, French, and German. See replace_months() in calibre's source calibre.utils.date.

I expect in your case the month abbreviations are in Spanish. In that language many 3-letter month abbreviations are the same as in English, but several differ from their English equivalents such as "abr" instead of "Apr"

You avoid this problem by using raw_field to get the date in ISO-format instead of localized. Something like this, replacing '#mydate' with the real lookup name:

{:'ifempty(format_date(raw_field('#mydate', ''), 'MMM'), 'Without date')'}

or this, which is more like your original.

{:'d = raw_field('#mydate', ''); test(d, strcat(format_date(d,'MMM')),'Without date')'}

Note: if this is the only template in the composite column then I suggest you use General Program Mode, as that will be faster because of template caching. Something like this:

program: ifempty(format_date(raw_field('#mydate', ''), 'MMM'), 'Without date')

Or even faster: a Python template:

python:
def evaluate(book, context):
	d = book.get('#mydate')
	if d is None:
		return 'Without date'
	# Localize these as you wish
	months = ['ene', 'feb', 'mar', 'abr', 'may', 'jun', 
		  'jul', 'ago', 'sep', 'oct', 'nov', 'dic']
	return months[d.month-1]