The .isValid() validation doesn't function correctly under the following condition:

• When the the day part is 31 for a month which didn't have 31 days.

Some instances where .isValid() returns true for a date that is invalid:

• dayjs().isValid('1995-02-31')
• dayjs().isValid('1995-03-31')
• dayjs().isValid('1995-04-31')
• dayjs().isValid('1995-06-31')
• dayjs().isValid('1995-09-31')
• dayjs().isValid('1995-11-31')