If changing how the publication date is displayed on the GUI is sufficient, then check the file 'tweaks.py' in your configuration folder.
You can open the configuration folder using preferences->advanced->Open calibre configuration directory. Once there, you should find the following in tweaks.py:
Code:
# Format to be used for publication date
# A string controlling how the publication date is displayed in the GUI
# d the day as number without a leading zero (1 to 31)
# dd the day as number with a leading zero (01 to 31)
# ddd the abbreviated localized day name (e.g. 'Mon' to 'Sun').
# dddd the long localized day name (e.g. 'Monday' to 'Qt::Sunday').
# M the month as number without a leading zero (1-12)
# MM the month as number with a leading zero (01-12)
# MMM the abbreviated localized month name (e.g. 'Jan' to 'Dec').
# MMMM the long localized month name (e.g. 'January' to 'December').
# yy the year as two digit number (00-99)
# yyyy the year as four digit number
# For example, given the date of 9 Jan 2010, the following formats show
# MMM yyyy ==> Jan 2010 yyyy ==> 2010 dd MMM yyyy ==> 09 Jan 2010
# MM/yyyy ==> 01/2010 d/M/yy ==> 9/1/10 yy ==> 10
# default if not set: MMM yyyy
gui_pubdate_display_format = 'MMM yyyy'
Set the variable to 'yyyy'.