I am using assetHistory to download data from Alpha Vantage using assetHistory(Name, FROM_AV)

By default it downloads all of the available data. I want to download the data for a more limited period, e.g. from (2020, 7, 21) to today.

According to the manual I should be able to do this by settings StartDate and EndDate. But when I set StartDate = 20200721 and EndDate = 2021, it makes no difference: assetHistory loads all of the available data, regardless.

In case it isn't possible to limit the amount of data downloaded, how can I truncate the data series, so that Zorro ignores the data prior to StartDate in calculating indicators?