I read, not sure where, that our perception of time is relative to our total experience of its passage. The more days you’ve had the shorter each new day appears to you. I’m not sure why that would be the case but it matches with my experience. Perhaps we filter out more of the details as our model of what is happening becomes more robust, so as we get older the brain is less attentive to the particular moments: it categorizes moments more effectively over time as a member of some set, saving resources but leading to this recognition of the time passing in that moment.