Informix - Need to create date time parameters for Where clause - informix

Question

Informix is not my normal environment and the way it handles datetime values is throwing me for a loop. I can't imagine this is difficult, but for the life of me I'm not yet able to figure it out.
This is the SQL:
SELECT agentid,
extension As Ext,
resourcefirstname As FirstNm,
resourcelastname As LastNm,
Min(eventdatetime) As FirstIn
FROM agentstatedetail AS asdr Join
resource As r On asdr.agentid = r.resourceid
WHERE asdr.eventdatetime BETWEEN '2016-10-20 04:00:00' AND '2016-10-21 03:59:59'
AND eventtype = 3
AND assignedteamid = 14
Group By agentid, extension, resourcefirstname, resourcelastname
Order By Min(eventdatetime)
Everything works as is, but the dates in the Between clause are currently entered manually- not optimal. I just need some way to describe "yesterday at 4:00 AM" and "Today at 4:00 AM" Will somebody please clue me in?

Answer 1

Using Informix version 12.10.FC6DE, I can do this:
SELECT
TODAY::DATETIME YEAR TO SECOND AS today_zerohour
, TODAY::DATETIME YEAR TO SECOND - '20:00:00'::INTERVAL HOUR TO SECOND AS yesterday_dawn
, TODAY::DATETIME YEAR TO SECOND + '04:00:00'::INTERVAL HOUR TO SECOND AS today_dawn
FROM
systables
WHERE
tabid = 1;
And it returns:
today_zerohour yesterday_dawn today_dawn
2016-10-21 00:00:00 2016-10-20 04:00:00 2016-10-21 04:00:00
So, what is happening here:
The operator TODAY returns the system date as a DATE type. The DATE type does not have the precision I want (it only has year, month and day), so I cast the value (cast operator is ::) to a DATETIME with precision from year to second (the hour, minutes and seconds are set to zero):
TODAY::DATETIME YEAR TO SECOND
In Informix, for an addition or subtraction with a DATETIME value to return another DATETIME value, I need to add or subtract an INTERVAL value. So I created 2 INTERVAL values.
One INTERVAL of 20 hours to subtract from the today value (again the cast operator :: is used, this time to cast from a string to an INTERVAL):
'20:00:00'::INTERVAL HOUR TO SECOND
One INTERVAL of 4 hours to add to the today value:
'04:00:00'::INTERVAL HOUR TO SECOND