I don't see much sense in calculating milliseconds

It's confusingly named, but delta_ms returns minutes+seconds, not milliseconds. It's the sister of delta_md, which returns months+days.

the upper and and lower limits are dates only, no time specified.

It really doesn't need to be said what's the first and last second of the day. Times were specified for the timestamps checked against the limits.

So yeah, it's still ambiguous. OP, what resolution do you want?