日子 vs 日期 - what is the difference between the two?
日期 = "date", whereas 日子 = "day".  Although in Chinese (as in English) they are sometimes interchangeable.

Ordinarily you'd expect a 日期 to have a numerical form:


  今天是什么日期？  今天是2020年4月15号。
  What is the date today?  Today is the 15-th April, 2020.


However, a 日子 isn't necessarily numerical:


  今天是什么日子？  今天是我的生日。
  What day is it today?  Today is my birthday.


We typically think of changing the 日期 ("date") rather than the 日子 ("day").


  你好，真不好意思，我要改一下日期，可以吗?
  Hello, I'm very sorry, I have to change my dates. Is that all right?
  ChinesePod  


We also typically think of having a good 日子 ("day"), rather than a good 日期 ("date").


  今天是个好日子。
  Today is a good day.