[drivers99]

    date: illegal option -- I
    usage: date [-jnRu] [-d dst] [-r seconds] [-t west] [-v[+|-]val[ymwdHMS]] ... 
                [-f fmt date | [[[mm]dd]HH]MM[[cc]yy][.ss]] [+format]

[yawaramin]

https://www.gnu.org/software/coreutils/manual/html_node/Opti...

[mynegation]

Let me guess, you are on a Mac.

[yawaramin]

Probably on an older Mac or FreeBSD. On my macOS 12.6, `man date` says:

    The -I flag was added in FreeBSD 12.0.

[drivers99]

Yup. macOS Catalina