You have two options.
Set the computeWithForm property on your datasource. Here's an example:
<xp:dominoDocument var="ds1" formName="example" computeWithForm="onsave"></xp:dominoDocument>
Change your date/time field on your XPage to include a converter that converts the field to a date field (and optionally include a date picker):
<xp:inputText id="dateField" value="#{ds1.dateField}"> <xp:this.converter> <xp:convertDateTime type="date"></xp:convertDateTime> </xp:this.converter> <xp:dateTimeHelper id="dateTimeHelper1"></xp:dateTimeHelper> </xp:inputText>