"Snap-To" 1 min time selected using shared time range picker in dashboard.

[jss]

I have a single visualization and line chart dashbord panels (there are more panels as well) which are linked to shared time range picker. In my case data is loaded into splunk after every 1 minute. How can I configure 'latest' time of these panels to snap to 1 min if I select Last 15 mins (earliest=now) from presets in time range picker so that it doesn't display '0' in panels? 
On selecting last 15 minutes from presets in shared time range picker 0 is displayed in single value and line chart visualization  

index=index1 sourcetype=sourcetype_name
| timechart span=1min sum(call_rate)

[ITWhisperer]

index=index1 sourcetype=sourcetype_name earliest="@m-15m" latest="@m-1m"
| timechart span=1min sum(call_rate)

[jss]

Actually the query is linked to shared time picker for whole dashboard. The xml looks like following:

<row>
    <panel>
      <single>
        <title>Call Rate</title>
        <search>
          <query>index=index1 sourcetype=sourcetype_name
| timechart span=1min sum(call_rate)</query>
          <earliest>$field1.earliest$</earliest>
          <latest>$field1.latest$</latest>
          <refresh>5m</refresh>
          <refreshType>delay</refreshType>
        </search>
        <option name="refresh.display">progressbar</option>
        <option name="trellis.enabled">0</option>
        <option name="useColors">1</option>
      </single>
    </panel>
</row>

I want if I select any time from time range picker, 'latest' time should snap to beginning of the minute. How can I do it using tokens in 'earliest' and 'latest'?

P.S: field1 is token for Time input of dashboard

[ITWhisperer]

Set a token in your timepicker change handler and use that instead

      <change>
        <eval token="tokTimeLatestStartOfMinute">relative_time(relative_time(now(),'latest')-1,"@m")</eval>
      </change>