Error ORA-01722 in SQL issue

Good Evening!
Please provide me a solution to this problem.
I was performing some addition and subtraction from dates in the SQL server database while this error occurred. I have faced this error for the first time. As far as I understand, it may be caused because I was adding non-numeric values along with the dates and was subtracting alphanumeric values from the dates. But it's my need that I have to add and subtract in this way.
I don’t know how SQL operates all arithmetic operations, but still I want someone to sort this out so I can add the values and numeric values my way and can get the desired output of numeric values.
Don’t stress on my idea ,but just try to figure it out that what may be the actual cause of this error. My work is stopped and will start when someone out of you people guides me about the solution of this problem. Waiting for your kind response can you solve it?

Error encountered An error was encountered performing the requested Operation.
ORA-01722: invalid number
01722. 00000 – “ invalid number”
*Cause:
*Action:
Vendor code 1722 Error at line:1 Column:131
