Time series - descriptive statistics

#1
My task is to summarises the descriptive statistics of time series data ( mean, SD , standard error ). It is fairly starightforward for a stationary series. But How do we find out the mean, standard deviation, standard error of the non stationary time series data?
I have been reading about the stationary series and how to make a time series stationary by differencing the series. If we take the mean of the differenced series, will that be a reflector of mean of the original data.
Any inputs on this?