Returns a type filter function that returns the given value if value is of type string | null and, in ISO
format, represents a valid date (if non-null), otherwise returns the given defaultValue.
Parameters
defaultValue: string|null = null
The value to return if the given value is not of type string | null or is not a valid ISO
date string.
Returns a type filter function that returns the given
valueifvalueis of typestring | nulland, in ISO format, represents a valid date (if non-null), otherwise returns the givendefaultValue.