DSWS .NET API - Get "Latest Value" of a Time Series

Hi all,

i'm using the .net API for DSWS available at
http://product.datastream.com/DswsClient/Docs/Downloads.aspx

I want to retrieve the last available value from a time series as of a specific date.

For example:

var request = new DSDataRequest()
     {
         Instrument = new DSInstrument("VOD"),
         DataTypes = new DSDataTypes("TRESGS"),
         Date = 
     };

How should I set the Date if i want to get only the last TRESGS value available before 2018-12-31?

Thanks,

Marco