Here's a sample request XML, where oldestUpdated value is the 19th of Jan and latest is 22nd Jan. All leads updated in this window of time (ie, between 2014-01-19T07:18:11.575-08:00 and 2014-01-22T07:18:11.557-08:00)will be returned in the response.
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<ns2:paramsGetMultipleLeads xmlns:ns2="http://www.marketo.com/mktows/">
<leadSelector xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="ns2:LastUpdateAtSelector">
<latestUpdatedAt>2014-01-22T07:18:11.557-08:00</latestUpdatedAt>
<oldestUpdatedAt>2014-01-19T07:18:11.575-08:00</oldestUpdatedAt>
</leadSelector>
<batchSize>150</batchSize>
<includeAttributes>
<stringItem>FirstName</stringItem>
<stringItem>AnonymousIP</stringItem>
<stringItem>Company</stringItem>
</includeAttributes>
</ns2:paramsGetMultipleLeads>