Listing 'at' jobs with custom time format

Hello I need to list 'at' jobs with the following format **"year-month-day hours:minutes:seconds"**. From the 'at' man page the synopsis I need is the following:

at [-V] -l [-o timeformat] [job ...]

where timeformat, according to the man page, is "strftime-like time format used for the job list". So I tried with the following command:

at -l -o "%y-%m-%d %H:%M:%S"

but keep getting: " at: invalid option -- 'o' "