The answer is to use the settings button (spanner/wrench and screwdriver) to the right of the time slider at the bottom of the map. This took me a while to discover.
advanced to set the necessary details.
You will then get the dialog box to configure your time slider.
One problem can be that the interval dropdown does not show all intervals. If the dropdown does not contain the extent you want, reduce / increase the time extent, the dropdown values will change, select your time interval in the dropdown and then set the correct extent. For example if it only shows week, months and years, reducing the time extent will show days so that they can be selected.
Further details are at http://server.arcgis.com/en/portal/latest/use/configure-time.htm