code: function toUnixTime() {
        if (this._cachedUnixTime !== null) {
          return this._cachedUnixTime;
        }
        var offset = this.utcOffset();
  
        // we use the offset trick to ensure
        // that we are getting the actual UTC time
        var ms = Date.UTC(
          this.year,
          this.month - 1,
          this.day,
          this.hour,
          this.minute,
          this.second - offset
        );
  
        // seconds
        this._cachedUnixTime = ms / 1000;
        return this._cachedUnixTime;
      }
docs: Converts the current instance to seconds since January 1st 1970.
  
  @return {Number}         Seconds since 1970