How-to-get-the-date-of-the-trading-day-of-a-symbol

I want to get the start date of a bar history. How can I do that? I tried bars.DateTimes[0] but this depends on the date range of the backtest. Is there a way to get the actual start date of the time series no matter what date range is selected?

I noticed that I missed "first" in the title of the post...