SQL Server 2019

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select null < '1900-01-01';
Msg 102 Level 15 State 1 Line 1
Incorrect syntax near '<'.

select null < '1900-01-01';
Msg 102 Level 15 State 1 Line 1
Incorrect syntax near '<'.

select null = '1900-01-01';
Msg 102 Level 15 State 1 Line 1
Incorrect syntax near '='.

select null <> '1900-01-01';
Msg 102 Level 15 State 1 Line 1
Incorrect syntax near '<'.

select null <> null;
Msg 102 Level 15 State 1 Line 1
Incorrect syntax near '<'.

select null = null;
Msg 102 Level 15 State 1 Line 1
Incorrect syntax near '='.

select '' >= '1900-01-01'
Msg 102 Level 15 State 1 Line 1
Incorrect syntax near '>'.

select '1900-01-01' >= ''
Msg 102 Level 15 State 1 Line 1
Incorrect syntax near '>'.

create table foo (
  time1 date,
  time2 date
);
insert into foo values (null, null), ('2020-01-01', null), (null, '2020-01-01');
3 rows affected

select
  time1, time2,
  case
  when time1 >= time2 then
    time1
  else
    time2
  end as without_isnull,
  case
  when isnull(time1, '1900-01-01') >= isnull(time2, '1900-01-01') then
    time1
  else
    time2
  end as with_isnull
from foo;


time1
time2
without_isnull
with_isnull


null
null
null
null

2020-01-01
null
null
2020-01-01

null
2020-01-01
2020-01-01
2020-01-01

time1	time2	without_isnull	with_isnull
null	null	null	null
2020-01-01	null	null	2020-01-01
null	2020-01-01	2020-01-01	2020-01-01