get minutes from selected hours

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i have following code for users to select hours.
its only for 1 to 12 hours format
all fields are dropdown

startHour : startMinutes : finishHour : finishMinutes : AM/PM

i want to validate with javascript that user can only select correct time. And calculate the minutes of selected hours from dropdown menus.
example

2:00 To 6:00 PM
3:15 To 4:45 PM
11:00(AM) To 2:00 PM

here is the code for my html

        	<table border="1" cellpadding="5" cellspacing="0" align="center">
            	<tr>
					<td>Start Time</td>
					<td>Finish Time</td>
					<td>AM/PM</td>
					<td>&nbsp;</td>
                </tr>
            	<tr>
					<td>
						<select id="cbo_start_hour" name="cbo_start_hour">
							<option value="-1">Hour</option>
							<option>1</option>
							<option>2</option>
							<option>3</option>
							<option>4</option>
							<option>5</option>
							<option>6</option>
							<option>7</option>
							<option>8</option>
							<option>9</option>
							<option>10</option>
							<option>11</option>
							<option>12</option>
						</select>
						<select id="cbo_start_minute" name="cbo_start_minute">
							<option value="-1">Minute</option>
							<option>00</option>
							<option>15</option>
							<option>30</option>
							<option>45</option>
						</select>
					</td>
					<td>
						<select id="cbo_finish_hour" name="cbo_finish_hour">
							<option value="-1">Hour</option>
							<option>1</option>
							<option>2</option>
							<option>3</option>
							<option>4</option>
							<option>5</option>
							<option>6</option>
							<option>7</option>
							<option>8</option>
							<option>9</option>
							<option>10</option>
							<option>11</option>
							<option>12</option>
						</select>
						<select id="cbo_finish_minute" name="cbo_finish_minute">
							<option value="-1">Minute</option>
							<option>00</option>
							<option>15</option>
							<option>30</option>
							<option>45</option>
						</select>
					</td>
					<td>
						<select id="cbo_am_pm" name="cbo_am_pm">
							<option value="-1">AM/PM</option>
							<option>AM</option>
							<option>PM</option>
						</select>
					</td>
					<td>
						<input type="button" value="Start Time" onClick="start_time()" class="btn_submit">
					</td>					
                </tr>
				
            </table>