AllAccepted answers
Robert Cemper · Jun 23, 2020

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.  

Robert Cemper · Jun 23, 2020

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

Robert Cemper · Jun 22, 2020

embedded SQL is probably an overkill for this formating.
Just convert it to the internal format, add what you need and assemble the pieces in COS

; set datein=201906192359
; set add=60    ;; seconds to add
set dh=$ZDH($e(datein,1,8),8)
set hh=$e(datein,9,10)
set mi=$e(datein,11,12)
set new=$ZTH(hh_":"_mi_":00",3)+add
set zdt=$zdt(new\86400+dh_","_(new#86400),3)
set dateout=$tr($e(zdt,1,*-3),"-: ")
Robert Cemper · Jun 16, 2020

As all you need is in %Persitent you could have your own personal.persitent extending %persitent
wrapping %Open and %Delete in your own extended code.
3 points to consider:

  • how to force your personal.persitent to all existent and future classes ?
  • how to force classes generated  by DDL ?
  • how to maintain your extension over release changes ?

Sorry, I doubt if this is the right approach

Robert Cemper · Jun 13, 2020

Allmost!  3rd Update

since if the left side is a Unix Timestamp, then you get this result:

write $zdt($zdth(1591891200000\1000,-2),3) ==> 2020-06-11 18:00:00

docs: https://docs.intersystems.com/iris20201/csp/docbook/Doc.View.cls?KEY=RCOS_fzdatetime 

timezone conversion from UTC may compensate  the 6 hours difference ! 
 in Middle Europ we have we have 2 hrs+  to UTC so

write $zdt($zdt($zdth(1591891200000\1000,-2),-3),3)
2020-06-11 16:00:00