I have a table with meter readings from every first day of the month. Each meter have unique ID. And I have a clients. The data of each customer is formed by a formula including its meters. The data for each meter are formed by a difference in readings. For example, data from 01.02 (subtraction) data from 01.01. I want to make a forum where to choose the client and the data from which dates to get. When choosing the date 01.05 to make the (subtraction) 01.05-01.04. I do not need to do it automatically. But I do not know how to make field choices.
may be something like this: