Today I got requirement to open new record form with prepopulated DateTime field. This article describes how to pre-set Date field:

The following sample sets the Est. Close Date field for a new opportunity to January 31, 2011. The unencoded value for the extraqs parameter is “estimatedclosedate=01/31/11”.

No single word about time component. Luckily I found a way to make it work. Format that should be used is following – M/d/yyyy H:m:s, something like to 1/31/2011 15:6:0.

Here is the code snippet you can use to open new task for prepopulating Due Date field with the same time as now plus 1 day:

var tomorrow = new Date();
tomorrow.setDate(tomorrow.getDate() + 1);

var formattedTomorrow = (tomorrow.getMonth() + 1) + "/" +
	tomorrow.getDate() + "/" +
	tomorrow.getFullYear() + " " +
	tomorrow.getHours() + ":" +
	tomorrow.getMinutes() + ":" +
	tomorrow.getSeconds();

var entityFormOptions = {
	entityName: "task"
};

var formParameters = {
	scheduledend: formattedTomorrow
};

Xrm.Navigation.openForm(entityFormOptions, formParameters);