Could you point how to correctly validate if the time before the date/ time filled on the textbox is greater or equal 03 hours?

Hi Experts

Could you point how to correctly validate if the time before the date/ time filled on the textbox is greater or equal 03 hours?

Accordingly to:

<td id="div_data_missao_ida_1" style="visibility: <?=(isset($guia['missao'][0]['nMissaoPreMarcada']) && $guia['missao'][0]['nMissaoPreMarcada']==1 ?"visible":"visible")?>;">
	<label class="form_title">Data da Miss&atilde;o:</label><br/>
	<input type="text"  class="input" name="dpremarcada_ida" id="dpremarcada_ida" value="<?=(isset($guia['missao'][0]['dMissao'])?formataStamp($guia['missao'][0]['dMissao'],"stamp","dma"):date("d/m/Y"))?>" size="11" maxlength="10"  onchange="copiaDataMissaoTIT();">
	<img src="imagens/calendario.png" name="cal_trigger" id="cal_trigger" border="0" align="absmiddle" onmouseover="javascript: calendar_param('dpremarcada_ida', 'cal_trigger', isAnterior);" style="cursor: pointer;" >
</td>


<td id="div_data_missao_ida_2" style="visibility: <?=(isset($guia['missao'][0]['nMissaoPreMarcada']) && $guia['missao'][0]['nMissaoPreMarcada']==1 ?"visible":"visible")?>;">
	<label class="form_title">Hor&aacute;rio:</label><br/>
	<input type="text"  onkeypress="campo_hora(event); return validaDigNum(event);" value="<?=(isset($guia['missao'][0]['dMissao'])?formataStamp($guia['missao'][0]['dMissao'],"stamp","hm"):date("H:i"))?>" size="5" maxlength="5" name="horario_premarcada_ida" id="horario_premarcada_ida" class="input" >            
</td>

The dtae/ time filled in the textbox must to be greater than 03 hours.

Could you suggest how to solve?

Thanks in advance