Let us you have the table with some duration fields, for example, field1 and field2. You can create Client Script with type onLoad, which has the following code:

function onLoad() {
	var doc = g_form.getControl("field1").ownerDocument,
		win = doc.defaultView,
		table = g_form.getTableName(),
		hideMinSecFields = function (field) {
			function getElem (field, suffix, prefix) {
				return doc.getElementById((prefix || "") + table + "." + field + (suffix || ""));
			}
			var controlHour = getElem(field, "dur_hour", "ni."),
				controlMin = getElem(field, "dur_min", "ni."),
				controlSec = getElem(field, "dur_sec", "ni."), cs;			

			if (controlMin != null && controlMin.style != null) {
				controlMin.style.display = "none";
			}
			if (controlSec != null && controlSec.style != null) {
				if (win != null && win.getComputedStyle != null) {
					cs = win.getComputedStyle(controlSec, null);
					if (controlHour != null && controlHour.style != null && cs != null) {
						controlHour.style.borderRightStyle = cs.getPropertyValue("border-right-style");
						controlHour.style.borderRightWidth = cs.getPropertyValue("border-right-width");
						controlHour.style.borderRightColor = cs.getPropertyValue("border-right-color");
						controlHour.style.borderBottomRightRadius = cs.getPropertyValue("border-bottom-right-radius");
						controlHour.style.borderTopRightRadius = cs.getPropertyValue("border-top-right-radius");
					}
				}
				controlSec.style.display = "none";
			}
		};
	hideMinSecFields("field1");
	hideMinSecFields("field2");
}

It's the same code, which I posted before, but where I posted before. The controls in the form will be displayed without min/sec. Something like on the picture below: