How to identify elements which are not part of web page in web testing


I am performing a web testing using appium java-client-5.0.0-BETA1. I want to select a date in my test case. When I click on the date icon a date picker appears where user can select appropriate date. This date picker is not the part of my web page. I inspected the element with UI Automator Viewer and extracted the correct xpath but still getting No Such Element Exception.

Appium Version - 5.0.0-BETA1
OS : Android 4.4.2
Device : Samsung Galaxy Tab

Below is the code I am using

            AppiumDriver driver = null;
	DesiredCapabilities capabilities = null;       
    	capabilities =;
		capabilities.setCapability(MobileCapabilityType.PLATFORM, Platform.ANDROID);    		
		capabilities.setCapability(MobileCapabilityType.DEVICE_NAME, "GalaxyTab");
		capabilities.setCapability(MobileCapabilityType.VERSION, "4.4.2");
		capabilities.setCapability(MobileCapabilityType.NEW_COMMAND_TIMEOUT, 300);   
    	driver = new AndroidDriver(new URL(""),capabilities);
            driver.findElement("selectDate")).click();              driver.findElement(By.xpath("//android.widget.FrameLayout/android.widget.Button[@text='Set']")).click();

Please help me to make this work.
Screen -