Solved! Go to Solution.
Map1.TimeExtent = New TimeExtent(Date.Now.AddDays(-90), Date.Now)
<esri:Map x:Name="MyMap" WrapAround="True" TimeExtent="{Binding ElementName=MyTimeSlider, Path=Value}"> <!-- more code goes here --> <esri:TimeSlider x:Name="MyTimeSlider" Height="20" TimeMode="TimeExtent" MinimumValue="{Binding ElementName=MyMap, Path=Layers[EarthquakesLayer].TimeExtent.Start, Mode=OneWay}" MaximumValue="{Binding ElementName=MyMap, Path=Layers[EarthquakesLayer].TimeExtent.End, Mode=OneWay}" Value="{Binding ElementName=MyMap, Path=Layers[EarthquakesLayer].TimeExtent, Mode=OneWay}" > </esri:TimeSlider>