Peter,
Setting the time in the service parameters did the trick. Thanks! Now if only I could get my data to display only one year at a time:
In Desktop, I can show exactly one year's worth of data by excluding either the start or end time of the current interval. In ArcGIS Online, showing data for one year at a time actually shows data from two years. Here are my settings:
Here is what my data looks like:
You can clearly see that the pink points (2016) are mixing with points from 2017 (navy) if those points fall on 1/1/2017. In ArcGIS Online, either the start or end date should be excluded by default with these settings. I consider this a BUG because the user is expecting 1 year of data but they get a year and one day.