Edit in JSFiddle

var app = new Vue({
  el: '#app',
  data: {
    release_date: '',
    platform: 0,
    result: "Pirma pasirink išleidimo datą",
    platforms: [
    	{
      	name: 'Windows',
        period: {
        	before: 7,
          after: 14
        }
      },
      {
      	name: 'Linux',
        period: {
        	before: 7,
          after: 14
        }
      },
      {
      	name: 'MacOS X',
        period: {
        	before: 7,
          after: 14
        }
      },
      {
      	name: 'iOS',
        period: {
        	before: 7,
          after: 1
        }
      },
      {
      	name: 'PlayStation 4',
        period: {
        	before: 3,
          after: 2
        }
      },
      {
      	name: 'PlayStation Vita',
        period: {
        	before: 3,
          after: 2
        }
      },
      {
      	name: 'Xbox One',
        period: {
        	before: 3,
          after: 1
        }
      },
      {
      	name: 'Android',
        period: {
        	before: 1,
          after: -1
        }
      },
      {
      	name: 'Nintendo Switch',
        period: {
        	before: 3,
          after: 1
        }
      },
    ]
  },
  methods: {
  	recalc: function () {
    	var date1 = new Date(this.release_date);
			var date2 = new Date();
			var timeDiff = date1.getTime() - date2.getTime();
			var diffDays = timeDiff / (1000 * 3600 * 24); 
      var good_period = (diffDays > 0)?this.platforms[this.platform].period.before:this.platforms[this.platform].period.after;
      var bad_msg = (!(diffDays > 0))? "Deja, jau nebe ;(": "Dar ne";
      if ((good_period == -1) || (good_period - Math.abs(diffDays) > 0)) {
      	this.result = "Taip, dar yra šansų!";
      } else {
      	this.result = bad_msg;
      }
    }
  },
  watch: {
  	release_date: function (dt) {
    	this.recalc();	
    },
    platform: function (platform) {
    	this.recalc();
    }
  }
});
<form method="post" id="app">
  <div>
    <label for="platform">Platforma</label>
    <select  id="platform" required v-model="platform">
      <option v-for="(platform, key) in platforms" :value="key">{{platform.name}}</option>
    </select>
  </div>
  <div>
    <label for="release_date">Išleidimo data</label>
    <input type="date" required id="release_date" v-model="release_date" />
  </div>
  <div>
  <label for="result">Atsakymas</label>
    <input type="text" required id="result" :value="result" readonly />
  </div>
</form>
#app {
  display: inline-table;
  max-width: 30em;
  font-family: 'Pacifico', cursive;
  & > div {  
    display: table-row!important;
    padding: 0.5em;
    width: 100%; 
    display: block;
    & > div {
      display: table-cell;
      text-align: center;
    }
    select, input {
      width: 100%;
      display: table-cell;
      padding: 0.5em;
      margin: 0.5em;
    }
    input {
      border-color: transparent;
      border-bottom-style: solid;
      border-bottom-width: 1px;
      border-bottom-color: lightblue;
      &:focus {
        outline: 0;
        border-bottom-color: blue;
      }
    }
    label {
      display: table-cell;
      padding-right: 1em;
      padding-left: 0.5em;
    }
    button {
      margin-left: 0 auto;
      border-color: lightblue;
      border-style: solid;
      background-color: transparent;
      padding: 0.5em;
      font-family: 'Pacifico', cursive;
    }
  }
}