Explicitly Set ValidFrom in Temporal Table - sql-server

Question

I'm trying to set the ValidFrom range for the current record in a temporal table. I'm doing this because I'm rebuilding history from another system (non SQL) into a data warehouse so the current version of records may be "as of" a date that's in the past. If I can't get this to work, my fall back is to add a row in the history table that fills in the gap but I'm thinking there's a way to get this to work. Maybe there are some ways with alter columns?
/******** CURRENT TIME=2018-03-10 15:32:26 *****/
CREATE TABLE TestHist(
ID int NOT NULL,
Name varchar(max),
--Temporal Stuff
ValidFrom datetime2(7) NOT NULL,
ValidTo datetime2(7) NOT NULL
)
GO
CREATE TABLE Test(
ID int IDENTITY(1,1) NOT NULL,
Name varchar(max),
--Temporal Stuff
ValidFrom datetime2(7) GENERATED ALWAYS AS ROW START NOT NULL,
ValidTo datetime2(7) GENERATED ALWAYS AS ROW END NOT NULL,
PRIMARY KEY CLUSTERED (ID ASC) ,
PERIOD FOR SYSTEM_TIME (ValidFrom, ValidTo)
)
WITH( SYSTEM_VERSIONING = ON ( HISTORY_TABLE = dbo.TestHist ) )
GO
ALTER TABLE Test SET (SYSTEM_VERSIONING = OFF)
go
--THIS WORKS BUT SETS THE VALIDFROM TO CURRENT TIME
insert into Test(name) values ('fred')
--AND BTW, THIS IS HOW I LOAD THE HISTORY (THIS WORKS TOO)
insert into TestHist(ID,Name,ValidFrom,ValidTo) values (1,'joe',null,'1/1/18','1/15/18')
insert into TestHist(ID,Name,ValidFrom,ValidTo) values (1,'steve','fred','2/1/18','3/1/18')
But the problem is that it sets the current ValidFrom time arbitrarily to when you do your insert statement:
select * from test
ID Name ParentName ValidFrom ValidTo
1 fred NULL 2018-03-10 15:32:26.4403041 9999-12-31 23:59:59.9999999
And here's what I wish I could do:
--THIS DOESN'T WORK
insert into Test(name,ValidFrom,ValidTo) values ('fred','2/1/18','9999-12-31 23:59:59.997')
I get this error:
Msg 13536, Level 16, State 1, Line 38
Cannot insert an explicit value into a GENERATED ALWAYS column in table 'CodeAnalytics.dbo.Test'. Use INSERT with a column list to exclude the GENERATED ALWAYS column, or insert a DEFAULT into GENERATED ALWAYS column.

Answer 1

You cannot update ValidFrom on the temporal table. However, you can update ValidFrom on the history table that keeps track of changes. You create a record there by changing any value in the temporal table.
So you can do following steps:
Change anything in rows of your temporal table where you want to change the value of the ValidFrom column. This step creates a record in the history table for every changed record in the original table.
Set system versioning off for your temporal table.
Update ValidFrom in your history table.
Set system versioning back on for your temporal table.

Answer 2

Edit: oops. it's 2019 now. anyway, i needed to do this, so leaving here in case anyone else finds useful.
maybe something like this is what you're looking for?
CREATE TABLE TestHist(
ID int NOT NULL,
Name varchar(max),
--Temporal Stuff
ValidFrom datetime2(7) NOT NULL,
ValidTo datetime2(7) NOT NULL
)
GO
CREATE TABLE Test(
ID int IDENTITY(1,1) NOT NULL,
Name varchar(max),
--Temporal Stuff
ValidFrom datetime2(7) GENERATED ALWAYS AS ROW START NOT NULL,
ValidTo datetime2(7) GENERATED ALWAYS AS ROW END NOT NULL,
PRIMARY KEY CLUSTERED (ID ASC) ,
PERIOD FOR SYSTEM_TIME (ValidFrom, ValidTo)
)
WITH( SYSTEM_VERSIONING = ON ( HISTORY_TABLE = dbo.TestHist ) )
GO
ALTER TABLE Test SET (SYSTEM_VERSIONING = OFF);
insert into TestHist(ID,Name,ValidFrom,ValidTo) values (1,'steve','2/1/18','3/1/18')
insert into TestHist(ID,Name,ValidFrom,ValidTo) values (1,'joe','1/1/18','1/15/18')
SET IDENTITY_INSERT Test ON;
insert into Test(id, name) values (1, 'fred')
--after dropping period one can update validfrom on temporal table from max history
alter table Test DROP PERIOD FOR SYSTEM_TIME;
GO
update test set validfrom =(select max(validto) from TestHist where id=test.ID);
--now add period and turn system versioning back on
alter table test ADD PERIOD FOR SYSTEM_TIME (ValidFrom, ValidTo);
GO
alter table test set (system_versioning = on (HISTORY_TABLE=dbo.TestHist));
GO
--think this gives what you want
select * from test for system_time all