What is the TimeMode you are using of the TimeSlider. By default the TimeSlider is set to cumulative which locks the minimum thumb to the start and allows the maximum thumb to freely move showing you everything from start to where you maximum thumb is on the slider. there are two other modes of the time slider, TimeInstant and TimeExtent. TimeInstant places both thumbs directly on top of each other only querying graphics that match the current DateTime down to the exact millisecond. the TimeExtent allows both thumbs to move freely letting you examine a range of dates.
Here is a link to one of the examples of working with time aware data and the time slider in our SDK
http://help.arcgis.com/en/webapi/silverlight/samples/start.htm#TimeFeatureLayer
Josh,
The Map.TimeExtent property is the range of dates you want to see. Each Graphic object has a TimeExtent property when the Graphic.TimeExtent property is within a valid range of the Map.TimeExtent property the graphic is made visible. In the example I linked the Map.TimeExtent property is bound to the TimeSlider control as the Thumbs move the TimeExtent property of the map is changed thus hiding or showing graphics on the map.