I saw this in your bug-request-list: reicast : do not ask date/time
But maybe this can be the solution...
Normally, but i didn't try this yet on Batocera, you can format and set and save clock to vmu from the settings page in the dreamcast bios.
If you insert for example an audio disc and start the dreamcast the disc isn't recognized as a gamedisc. It will show you a cd player interface from here you go back one page. From here you can format your VMU's and set date and time and you can save date and time to your VMU.
From this point the dreamcast won't ask you to enter the date and time on every start.
(not tested yet on Batocera, but works on reicast on other platforms)