mysql 5.6 is Not referring Indexes for deterministic function DATEDIFF I do not understand why MySQL is not using the index for the below query.

I have a table named users and its create table is 



this table contains data.

Now I am running SQL select statements and it's checking its query execution plan.

for the statement 

desc select *
     from   users
     where  users.created_at between '2019-05-04' and '2019-05-06';


output is 

but which I am using DateDiff function query is not using indexes.

desc select *
     from   users
     where  DATEDIFF(users.created_at,'2019-05-05 23:11:56.547') =  0;


and output for this is 

according to the MySQL documentation page "A function is nondeterministic if, given fixed values for its arguments, it can return different results for different invocations".

According to doc, DateDiff is a deterministic function and I am not understanding why MySQL is not taking indexes for the query.
Reason for indexes not being used is table column is an input part of the function.
Current SQL Query

select *
 from   users
 where  DATEDIFF(users.created_at,'2019-05-05 23:11:56.547') =  0;


Indexes will be used if the query is in below way

select *
 from   users
 where  users.created_at =  DATE_ADD('2019-05-05 23:11:56.547', INTERVAL 0 DAY);


since above refers to a point of time. To check day we can have to use range timestamp start of today to start off tomorrow,

select *
 from   users
 where  users.created_at between DATE_ADD('2019-05-05 ', INTERVAL 0 DAY)
and DATE_ADD('2019-05-05 ', INTERVAL 1 DAY);