Date incompatability

Other SQL Server Topics
1

I have three date fields that I am trying to subtract and get a number of years. Two of the are dob and one is just date entered, I only need to use one of the dob fields, but I cannot get the subtract to work on any of them
1st date is - dob (varchar(15), null)
2nd date is - date (datetime, null)
3rd date is tdob is - (datetime, notnull)
my issue is when I try to subtract the two dates I get this error.
Arithmetic overflow error converting expression to data type datetime.
If I convert the two dates to varchar(10),101 format and try to subtract I get this error.
The data types varchar and varchar are incompatible in the subtract operator.
If I use the tdob field I get
Arithmetic overflow error converting expression to data type datetime.
If I convert only the tdob to Varchar(10) and not the date field,
Arithmetic overflow error converting expression to data type datetime.
If I convert both to Varchar(10) I get this error
The data types varchar and varchar are incompatible in the subtract operator
And last if I only conver the date field and not the tdob field
Arithmetic overflow error converting expression to data type datetime.

I know this is confusing, but I really need to get the age of the person between these two dates.

2

The error is in trying to convert the varchar to a datetime (probably) and you probably need to do this to subtract dates.
What format is the date in that column/variable?
Best is yyyymmdd hh:mm:ss or just yyyymmdd as they will implicitly convert.
Otherwise you need to do a convert(datetime,xxxx,style), look up the style in the cnvert statement.
Or you can use string functions to convert to yyyymmdd.
Then the subtract will work.
If any of the fields in the expression is a datetime it will try to convert to a datetime

3

Original fields are
dob date
10/30/1945 2017-12-11 00:00:00.000

4

dob
10/30/1945

date
2017-12-11 00:00:00.000

5

2nd dob field is this
tDob
1945-10-30 00:00:00.000

6

I finally figured it out.