How to get the time difference in hours between 2 dates where dates are stored in 12 hour format in Oracle?

I have two dates : Date1: 20-Mar-2020 07:35:00 PM and Date2: 20-Mar-2020 02:42:00 PM

I have written the query :

SELECT     ROUND(minutes / 60, 2) || ' Hours ' 
FROM     ( SELECT  (Date1 - to_date(Date2, 'DD-MON-YYYY HH12:MI:SS')) * 24 * 60     AS minutes
  FROM     dual
);

But I am not getting the desired result it is displaying 19.58 hours which is wrong , please correct me where am I going Wrong ?

Answer

As commented by Mat, you need to convert both strings to date, then substract them:

to_date(date1, 'dd-mon-yyyy hh12:mi:ss am') - to_date(date2, 'dd-mon-yyyy hh12:mi:ss am')

Note that your format specifier was missing the am part.

This gives you a result as a decimal number of days, that you can then convert th hours by multiplying it by 24:

round(
    ( 
        to_date(date1, 'dd-mon-yyyy hh12:mi:ss am') 
        - to_date(date2, 'dd-mon-yyyy hh12:mi:ss am')
    ) * 24, 
    2
) || ' hours'

Demo on DB Fiddle:

select round(
    (
        to_date('20-Mar-2020 07:35:00 PM', 'dd-mon-yyyy hh12:mi:ss am') 
        - to_date('20-Mar-2020 02:42:00 PM', 'dd-mon-yyyy hh12:mi:ss am')
    ) * 24, 
    2
) || ' hours' res
from dual

| RES        |
| :--------- |
| 4.88 hours |