There's currently not an easy way to calculate the time elapsed between two bson::DateTimes without converting to other types. We should implement subtraction methods to do this; we can likely copy checked_sub, saturating_sub, and impl Sub from OffsetDateTime: https://docs.rs/time/latest/time/struct.OffsetDateTime.html#

Edit: a more accurate analogy would be the checked_duration_since and saturating_duration_since methods in std::time::Instant.