Eligible features which can be used in energy forecasting

Can my features have an increasing trend?Will it violate my eligibility criteria?
Only year cannot be used in the calculation of any feature right?

If you fit your increasing trend on future data, it is not eligible.

could you explain with an example other than year?

If you use a trend at prediction time, that trend should be calculated on the data which is in the past when you make that prediction.

So, features like days since the start of the training period are not allowed, right?

This means i can use past years in my calculation of features.Suppose i have from year 2015 in test data,then i can use upto year 2014 in my feature calculations right?