If you are using only one time field to visualize your data over time, you might find that there are gaps in the data displayed if your data is not regularly spaced. This may result in a period when nothing is displayed.
To overcome this, you can calculate an end time field where the time value for a particular feature is the start time value of the next feature. The Calculate End Time tool can be run to calculate the end time field.
A feature with a start and end time can be thought of as a feature valid for a certain duration. When visualizing your data using the time slider, the feature will be displayed till the time on slider is within the start and end time of the feature. For example, if the start time of a feature in your data is January 1, 2010 and the end time is January 15, 2010, the feature will start displaying as soon as the time slider's time is January 1, 2010. The feature is displayed till January 15, 2010.