When using Splunk's dashboard editor, shared timepicker is not an available option for dynamic searches on other inputs such as dropdowns and multiselects. This can be overwritten by incorporating the shared timepicker within the search string i.e. earliest=$global_range.earliest$ latest=$global_range.latest$ but it would be nice to have the option similar to that of the statistics table that the inputs are being used to populate.
After you have add input time range with this source code:
<input type="time" token="global_range" searchWhenChanged="true">
<label/>
<default>
<earliestTime>0</earliestTime>
<latestTime>now</latestTime>
</default>
</input>
Add this source code after each searches string that you have in your dashboard
<earliestTime>$ global_range.earliest$</earliestTime>
<latestTime>$ global_range.latest$</latestTime>
And time range will be applied to each panel on your dashboard.
This should be added to GUI, since it already there for Panel.