Store a Value and use it for a calc

[Tuwat47]

You cannot look in to the future, not yet
What is the period you want the data for?

I tought LM was able to divine the future

Sorry Daniel my was only an example.

I need to store data from now to indefinite date. I use this data for a script you have already seen this morning. The first day of every month I need to send an email to my customer using LM (I already know how to do it) where I show him the consumption of the month that has just ended. For achieve this result I need to subtract the energy at the beginning of the month from the energy at the end of the month.

Hope to be clear in this post. I'm sure it will be useful for a lot of newbie users like me.