Timestamp across DB versions

So the other day I decided to write up an alert to let us know when a DB crashes, and nobody hears it, but I was struggling with joining our instances for some unknown reason due to this error message:

ORA-30175: invalid type given for an argument

I shelved it for a day or two until I had a chunk of time to dig into this a bit deeper, and with the help of an Oracle forums post I realized what my problem was.  You see, the source of my confusion resided in the fact that the below script was able to be run individually for each instance and I only had problems when starting to union them together:

select 'INST1' AS "SERVER", process_id, host_id, substr(reason, 0, 40) AS STATUS, creation_time from DBA_ALERT_HISTORY@inst1
where creation_time > sysdate -1
and object_type = 'INSTANCE'
union all
select 'INST2' AS "SERVER", process_id, host_id, substr(reason, 0, 40) AS STATUS, creation_time from DBA_ALERT_HISTORY@inst2
where creation_time > sysdate -1
and object_type = 'INSTANCE'

Looks benign right?  Well I realized that I was joining 11g and 10g DB tables together and that the timestamp object has apparently changed a bit, so that even with the table definitions not changing our system just wouldn't let me join the tables together no matter what I did when I do a value comparison on creation_time!  Finding that post was the key to getting this changed, because when I updated the script to be:

select 'INST1' AS "SERVER", process_id, host_id, substr(reason, 0, 40) AS STATUS, creation_time from DBA_ALERT_HISTORY@inst1
where creation_time > to_timestamp(trunc(systimestamp))
and object_type = 'INSTANCE'
union all
select 'INST2' AS "SERVER", process_id, host_id, substrDBA_ALERT_HISTORY@inst2
where creation_time > to_timestamp(trunc(systimestamp))
and object_type = 'INSTANCE'

everything was right with the world, and no more error messages were returned.  Oddly enough, or not as I've already explained, the same script with creation_time > to_timestamp(trunc(systimestamp)) -1 starts to bomb out again because the timestamp object is again having a comparison done to it.