The most simple way to do it:
in  class Table.A have 
       Property TableB as Table.B;
in  class Table.B have
                  Property TableA as Table.A;

in your code it may look like this:

set objA=##class(Table.A).%OpenId(212)
set objB=##class(Table.B).%OpenId(99)
set objA.TableB=objB
set objB.TableA=objA
do objA.%Save(), objB.%Save()

You are free to index properties TableA or TableB according to your needs
and you can also use Implicit JOIN between these tables.  

OK. I understand those records where created BEFORE you defined your new column.
I checked the internal code

$s(%val="":"",1:$zdatetime($zdTH($zdatetimeh(%val,3,,,,,,,,0),-3),3,,3))

This tells me:
- Your value is Not Empty 
- it has no valid time format, so it is trapped by the ERROR option and set to Zero  ==>> 1840-12-31 00:00:00

Suggestion:
- set value to NULL for old records.
or
- create your own data type for this case. 
or

- use a calculated Property  to trap the values