Ideally you wouldn't want to impose the requirement of ```git_buf``` just for getting a date, IMHO the dependency/layering wouldn't be correct. Sure I'm stuffing it into a ```git_buf``` outside, that doesn't mean that for every RFC2822 date you want to use a ```git_buf```. Adding a ```#include "buffer.h"``` inside ```utils.c``` also doesn't feel right.

An RFC2822 date is anyway fixed in length, incurring a possible extra 2/3 memory allocations and moves (depending on the buffer growing scheme used) is really not necessary inside the date function.