Debian distribution: bookworm
Linux 5.16.0-3-amd64 #1 SMP PREEMPT Debian 5.16.11-1 (2022-02-25) x86_64 GNU/Linux
libc6 2.33-7

xls file with a column of dates (german format) with the content:

01.01.2000
01.01.2010
01.01.2020

xls2csv converting to tab separated file and date in iso format:

$ xls2csv -c $'\t' -q 0 -f '%Y-%m-%d' dates.xls > dates.tsv

the resulting file contains:

$ cat dates.tsv
2004-01-02
2014-01-02
2024-01-02

All converted dates got added 4 years and 1 day

Thank you
Best wishes