小程序开发指南文档4.6.4小节缓存 用户登录态SessionId
如果登录状态保持一天，在小程序启动时，只需判断now - expiredTime <= 0 即可
App({

  onLaunch: function(options) {

    var sessionId =wx.getStorageSync('SESSIONID')

    var expiredTime =wx.getStorageSync('EXPIREDTIME')

    var now = +new Date()



    if (now - expiredTime <=1*24*60*60*1000) { // 此处有错误

      this.globalData.sessionId = sessionId

      this.globalData.expiredTime = expiredTime

    }

  },

  globalData: {

    sessionId: null,

    expiredTime: 0

  }

})