How to query meter readings

Chrisss

I am building a php web site that allows users to enter meter readings and then analyse those readings and then calculate energy costs.

I am not sure how to approach the analysis query as the user may not have entered a reading for every date and therefore may enter a date where a reading does not exist in the database.

Is there a way that php or mysql can return the closest date and the reading that relates to that.

Thanks