Don't use 'oldtime' feature for Chrono

Hi! Chrono, by default, enables the "oldtime" feature, which imports the time crate version 0.1.43

This crate has a Rustsec vulnerability, so it shouldn't be enabled by default IMO. As far as I can tell, this crate doesn't actually need that feature anyway. You can just use the "std" and "clock" features explicitly.