I have a date field called `BIRTH_DAT` and the date of births have been recorded like this:

`YYYMMDD`, where the second number of the year is missing.

So an example would be a date of birth of `4th March 1978` would appear in this field as: `1780304`

The rule in this field is that if it begins with a '1' then the date is in the 1900s, if it begins with a '2' then the date is in the 2000s

So what I want to do is to create another column that shows the correctly written version of the date so I can calculate age from it.

e.g 

* Column 1 is called `BIRTH_DAT` and has values:
1801204, 1601228, 1980803 ...
* Column 2 is a new column called `PROPER_DOB` and has values:
19801204, 19601228, 19980803 ...

How do I go about this?