So really it's R + your computer's localethe C function ctime that's responsible for the date format You can edit the snippet to have it generate any date/time format you like by going to Preferences > Code > look for the tiny button at the bottom of the pane that says "Edit Snippets...".
Admittedly, the snippet editing area could be easier to find!
Snippets are a lot of fun... go forth and wield this mighty new power
I'm totally with you on the oddness of that particular datetime format! But it seems to be venerable (see edit above).
Now I'm wondering... who chose that—of all things—as a fixed string datetime representation, and why?? Sadly, cursory googling has not revealed anything like "an oral history of the genesis of C's time library", and this is getting seriously off-topic, so I think that's far enough down the hole for me!