Remember that the DateTime module only thinks in terms of absolute timeranges. It would have no idea what to do with relative timeranges like "last 24 hours", whose representation in Splunk look something like "-24h@h,@h".
However... yes. Bear with me.
DateTime doesn't have any direct param to set a default value, but like all Sideview form element modules, if it sees a key coming from upstream whose name matches its "name" param, it will try to set itself to that value.
[[CORRECTION : DateTime is actually the exception to this rule. The DateTime module's name param can only be "earliest" or "latest", and it instead looks at the Splunk.TimeRange object that comes down, rather than the raw $earliest$ or $latest$ key. As a result we need to also use a weird trick to turn our $earliest$ and/or $latest$ keys into a timerange.
<module name="Search">
<param name="earliest">$earliest$</param>
<param name="latest">$latest$</param>
]]
1) So a really simple example would be setting it from the URL.
In this page, if the URL had `viewName?earliest=1394836434
<module name="URLLoader" autoRun="True">
<module name="DateTime">
<param name="name">earliest</param>
<param name="label">Search all events after</param>
then the DateTime module would set itself to that time.
(Technically it's not looking at the $earliest$ but rather the TimeRange object put in there by URLLoader)
2) Another simple example would be to set it from a ValueSetter module.
<module name="ValueSetter" autoRun="True">
<param name="arg.earliest">1394836434</param>
<module name="Search">
<param name="earliest">$earliest$</param>
<module name="DateTime">
<param name="name">earliest</param>
<param name="label">Search all events after</param>
3) And anything you can set from a ValueSetter you can of course set from a ResultsValueSetter module. Meaning that you can have a search that returns time values in epochtime format (seconds since 1/1/1970), and yes you can technically compose a search or postprocess that can get for you the epochtime values of any other timerange, including relative timeranges. Remember to use that Search module to turn the raw key into a timerange and you can do quite a lot as far as prepopulating DateTime modules to dates dynamically.
So for instance in this example the DateTime modules would be populated by default with the absolute timerange of "last 24 hours".
<module name="Search" layoutPanel="panel_row2_col1" autoRun="True">
<param name="search">| stats count | fields - count | addinfo | rename info_min_time as earliest info_max_time as latest</param>
<param name="earliest">-24h@h</param>
<param name="latest">@h</param>
<module name="ResultsValueSetter">
<param name="fields">earliest, latest</param>
<module name="Search">
<param name="earliest">$earliest$</param>
<param name="latest">$latest$</param>
<module name="DateTime">
<param name="name">earliest</param>
<param name="label">From</param>
<module name="DateTime">
<param name="name">latest</param>
<param name="label">To</param>
...
</module>
</module>
</module>
</module>
</module>
The | stats count | fields - count | addinfo | rename info_min_time as earliest info_max_time as latest search is a peculiar business. It's a search that doesn't actually search for anything. It's only purpose is to talk to Splunkd very very quickly, so as to get the addinfo command to convert the timerange for us... Lots of strange little tasks can be washed through the splunk search language where ordinarily you'd have to write some custom javascript code. your mileage of such things my vary. And vary very much in proportion to your love of maintaining custom code. 😃
