how to make label visible/invisible?

I have these date and time fields, and I want to set a javascript validation for the time.

If the format is invalid, it should make the label visible, else it should be invisible.

This is the code I have so far.

  <td nowrap="nowrap" align="left">
            <table border="0" cellpadding="0" cellspacing="0">
                <tr>
                    <td align="right" nowrap="nowrap">
                        <span id="startDateLabel">Start date/time: </span>
                        <input id="startDateStr" name="startDateStr" size="8" onchange="if (!formatDate(this,'USA')) {this.value = '';}" />
                        <button id="startDateCalendarTrigger">...</button>
                        <input id="startDateTime" type="text" size="8" name="startTime" value="12:00 AM" onchange="validateHHMM(this.value);"/>
                        <label id="startTimeLabel" visible="false">Time must be entered in the format HH:MM AM/PM</label>
                        <BR>
                        <span id="endDateLabel">End date/time: </span>
                        <input id="endDateStr" name="endDateStr" size="8" onchange="if (!formatDate(this,'USA')) {this.value = '';}" />
                        <button id="endDateCalendarTrigger">...</button>
                        <input id="endDateTime" type="text" size="8" name="endTime" value="12:00 AM" onchange="validateHHMM2(this.value);"/>
                        <label id="endTimeLabel" visible="false">Time must be entered in the format HH:MM AM/PM</label>
                    </td>
                </tr>
            </table>
        </td>

The problem is the label shows up when loaded irrespective of what I set as visible. I tried visibility = "hidden" and it still shows up.

Here is the validation part:

    <