Write a program to count the number of days between two dates.

The two dates are given as strings, their format is `YYYY-MM-DD` as shown in the examples.

Example 1:

    Input: date1 = "2019-06-29", date2 = "2019-06-30"
    Output: 1

Example 2:

    Input: date1 = "2020-01-15", date2 = "2019-12-31"
    Output: 15

Constraints:

- The given dates are valid dates between the years `1971` and `2100`.