Foreigners often make some strange mistakes in Chinese when they study the most difficult language--- ---Chinese. We try to analyze typical sentences to correct these errors.

         Jīntiān shì 3 yuè 20 tiān.
False: 今天是3月20天。
        Jīntiān shì 3 yuè 20 hào.
True: 今天是3月20号。

“天”和“号”混淆，应改为“3月20号”，口语里常用。书面语里还可以用“日”。
The speaker has confused the word 天 with 号 which is the proper word to be used here. An alternative to 号 is 日. But 号 is frequent in spoken Chinese whereas 日 is often seen in written Chinese.