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HTML 

              
                <!DOCTYPE html>
<html lang="pt-br">

<head>
    <title>Calculadora</title>
    <meta charset="UTF-8">
    <meta http-equiv="content-type" content="text/html;" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <!-- BOOTSTRAP -->
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" integrity="sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm" crossorigin="anonymous">
    <!-- google fonts -->
    <link href="https://fonts.googleapis.com/css?family=Press+Start+2P" rel="stylesheet">
    <link href="https://fonts.googleapis.com/css?family=Lobster" rel="stylesheet">

    <!-- javaScript -->
    
    <script type="text/javascript" src="js/calculadora.js"></script>

</head>

<body>
    <style>
        
    </style>
    <div class="container">
        <div class="row">
            <div class="col-xs-12" id="titulo">
                <h3 >Calculadora Básica</h3>
            </div>
        </div>
    </div>
    <div class="container">
        <div class="row align-items-center">
            <div class="col-xs-12 col-sm-6 col-md-3 fundo-mod margem borda">
                    
                <div class="justify-content-center centro">
                    
                    <table class="table table-dark">
                        <tr>
                            
                            <td colspan="4" ><div id="historico" onclick="mostrarHora();"></div></td>
                        </tr>
                        <tr>
                            
                            <td colspan="4"><input type="text" name="visor" id="visor" class="col-xs-12 col-sm-12 col-md-12" /></td>
                        </tr>
                        <tr>
                            <td><input type="button" id="botao7" value="7" class="btn btn-secondary mesmo-tamanho" onclick="botao(value);"/></td>
                            <td><input type="button" id="botao8" value="8" class="btn btn-secondary mesmo-tamanho" onclick="botao(value);"/></td>
                            <td><input type="button" id="botao9" value="9" class="btn btn-secondary mesmo-tamanho" onclick="botao(value);"/></td>
                            <td><input type="button" id="botao/" value="/" class="btn btn-info mesmo-tamanho" onclick="btn_divide(value);" /></td>
                        </tr>
                        <tr>
                            <td><input type="button" id="botao4" value="4" class="btn btn-secondary mesmo-tamanho" onclick="botao(value);"/></td>
                            <td><input type="button" id="botao5" value="5" class="btn btn-secondary mesmo-tamanho" onclick="botao(value);"/></td>
                            <td><input type="button" id="botao6" value="6" class="btn btn-secondary mesmo-tamanho" onclick="botao(value);"/></td>
                            <td><input type="button" id="botao*" value="*" class="btn btn-info mesmo-tamanho" onclick="btn_multiplica(value);"/></td>
                        </tr>
                        <tr>
                            <td><input type="button" id="botao1" value="1" class="btn btn-secondary mesmo-tamanho" onclick="botao(value);"/></td>
                            <td><input type="button" id="botao2" value="2" class="btn btn-secondary mesmo-tamanho" onclick="botao(value);"/></td>
                            <td><input type="button" id="botao3" value="3" class="btn btn-secondary mesmo-tamanho" onclick="botao(value);"/></td>
                            <td><input type="button" id="botao-" value="-" class="btn btn-info mesmo-tamanho" onclick="btn_subtrai(value);" /></td>
                        </tr>
                        <tr>
                            <td><input type="button" id="botao0" value="0" class="btn btn-secondary mesmo-tamanho" onclick="botao(value);"/></td>
                            <td><input type="button" id="botao=" value="=" class="btn btn-info mesmo-tamanho" onclick="btn_igual(value);"/></td>
                            <td><input type="button" id="botaoC" value="C" class="btn btn-danger mesmo-tamanho" onclick="reset(value);"/></td>
                            <td><input type="button" id="soma" value="+" class="btn btn-info mesmo-tamanho" onclick="btn_soma(value);"/></td>
                        </tr>
                    </table>
                </div>
                
            </div>
        </div>
    </div>
</body>

</html>
!

CSS 

              
                
  .fundo-mod {
    background-color: rgb(23, 162, 184);
        
  }

  .margem {
    padding: 1%;
    border-radius: 5px;
    
  }
  .mesmo-tamanho {
    border-radius: 1px;
    
}

.display {
    margin: 0;
    position: absolute;
    top: 50%;
    left: 50%;
    margin-right: -50%;
    transform: translate(-50%, -50%);
}
#visor{
  font-family: 'Press Start 2P', cursive;
}
#historico{
  font-family: 'Press Start 2P', cursive;
}
#titulo{
  font-family: 'Lobster', cursive;
}
.borda{
  -webkit-box-shadow: 0 3px 9px black;
  box-shadow: 0 3px 9px black;
  border-radius: 3px;
  -webkit-transition: border .2s ease-in-out;
  transition: border .2s ease-in-out;
}

.centro {
  width:100%;
  height:100%;
  }
!

JS 

              
                var valorVisor = 0;
var numeroA;
var numeroB;
var operacao;
var agora = new Date;


function botao(dado) {
    var auxiliar = document.getElementById("visor").value; // auxiliar recebe o valor pressionado no visor
    document.getElementById("visor").value = auxiliar + dado; // visor recebe o valor de auxiliar e concatena com dado


    valorVisor = document.getElementById("visor").value = auxiliar + dado;
    //document.getElementById("historico").innerHTML = agora.getHours();
    
    // mostrar a saudação acima do visor
    var hora = agora.getHours();
    
    if(hora >= 0 && hora <= 12){
      document.getElementById("historico").textContent = "Bom dia";
    }
    if(hora >= 13 && hora <= 17){
      document.getElementById("historico").textContent = "Boa tarde";
    }
    if(hora >= 18 && hora <= 23){
      document.getElementById("historico").textContent = "Boa noite";
    }
}


function btn_soma(caracter){
    numeroA = valorVisor;
    operacao = "+";
    
    limpar();
    //document.getElementById("historico").innerHTML += operacao;
}

function btn_subtrai(caracter){
  numeroA = valorVisor;
  operacao = "-";
  
  limpar();
}

function btn_multiplica(caracter){
  numeroA = valorVisor;
  operacao = "*";
  
  limpar();
}

function btn_divide(caracter){
  numeroA = valorVisor;
  operacao = "/";
  
  limpar();
}

function reset() {
    // limpar visor
    document.getElementById('visor').value = '';
    valorVisor = 0;
    operacao = "";
}
function limpar(){
    document.getElementById('visor').value = '';
    
}

function btn_igual(){
    numeroB = valorVisor;
    calcular();
}

function calcular() {
    // faz o calculo, pega o resultado e colocar no visor
    //document.getElementById('visor').value = eval(resultado);

    //document.getElementById('visor').value = resultado;
    //document.getElementById('visor').value = eval(valorVisor);

    var total = 0;
    var ultimoTotal = 0;
      switch(operacao){
        case "+":
          total = parseFloat(numeroA) + parseFloat(numeroB);
          break;
        case "-":
            total = parseFloat(numeroA) - parseFloat(numeroB);
            break;
        case "*":
          total = parseFloat(numeroA) * parseFloat(numeroB);
          break;
        case "/":
          total = parseFloat(numeroA) / parseFloat(numeroB);
          break;
      }
      ultimoTotal = total;
      reset();
      document.getElementById('visor').value = total;
      valorVisor = ultimoTotal;

}
!
999px

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