Please note that there is useful information at SO 440061 about converting between 12 hour and 24 hour notations for time (the opposite of this conversion); it isn't trivial, because 12:45 am comes half an hour before 1:15 am.
Next, please note that IDS version 7.31 finally reached its end of service on 2009-09-30; it is no longer a supported product.
You should be more precise with your version number; there are considerable differences between 7.30.UC1 and 7.31.UD8, for instance.
However, you should be able to use the TO_CHAR() function to format times as you need. Although this reference is to the IDS 11.50 Information Center, I believe that you will be able to use it in 7.31 (not necessarily in 7.30, but you should not have been using that for most of the last decade).
There is a '%R' format specifier for 24-hour time, it says. It also refers you to 'GL_DATETIME', where it says '%I' gives you the 12-hour time and '%p' gives you the am/pm indicator. I also found a 7.31.UD8 instance of IDS to validate this:
select to_char(datetime(2009-01-01 16:15:14) year to second, '%I:%M %p')
from dual;
04:15 PM
select to_char(datetime(2009-01-01 16:15:14) year to second, '%1.1I:%M %p')
from dual;
4:15 PM
I see from re-reading the question that you actually have SMALLINT values in the range 0000..2359 and need to get those converted. Often, I'd point out that Informix has a type for storing such values - DATETIME HOUR TO MINUTE - but I concede it occupies 3 bytes on disk instead of just 2, so it isn't as compact as a SMALLINT notation.
Steve Kass showed the SQL Server notation:
select
cast((@milTime/100+11)%12+1 as varchar(2))
+':'
+substring(cast((@milTime%100+100) as char(3)),2,2)
+' '
+substring('ap',@milTime/1200%2+1,1)
+'m';
The trick for getting the hour correct is neat - thanks Steve!
Translated into Informix for IDS 11.50, assuming that the table is:
CREATE TEMP TABLE times(begin_tm SMALLINT NOT NULL);
SELECT begin_tm,
(MOD(begin_tm/100 + 11, 12) + 1)::VARCHAR(2) || ':' ||
SUBSTRING((MOD(begin_tm, 100) + 100)::CHAR(3) FROM 2) || ' ' ||
SUBSTRING("ampm" FROM (MOD((begin_tm/1200)::INT, 2) * 2) + 1 FOR 2)
FROM times
ORDER BY begin_tm;
The SUBSTRING notation using FROM and FOR is standard SQL notation - weird, but so.
Example results:
0 12:00 am
1 12:01 am
59 12:59 am
100 1:00 am
559 5:59 am
600 6:00 am
601 6:01 am
959 9:59 am
1000 10:00 am
1159 11:59 am
1200 12:00 pm
1201 12:01 pm
1259 12:59 pm
1300 1:00 pm
2159 9:59 pm
2200 10:00 pm
2359 11:59 pm
2400 12:00 am
Caution: the values 559-601 are in the list because I ran into a problem with rounding instead of truncation in the absence of the cast to integer.
Now, this was tested on IDS 11.50; IDS 7.3x won't have the cast notation.
However, that isn't a problem; the next comment was going to deal with that...
As an exercise in how to write the expression in SQL without conditionals, etc, this is interesting, but if anyone wrote that more than once in an entire suite, I'd shoot them for lack of modularization. Clearly, this requires a stored procedure - and a stored procedure doesn't need the (explicit) casts or some of the other trickery, though the assignments enforce implicit casts:
CREATE PROCEDURE ampm_time(tm SMALLINT) RETURNING CHAR(8);
DEFINE hh SMALLINT;
DEFINE mm SMALLINT;
DEFINE am SMALLINT;
DEFINE m3 CHAR(3);
DEFINE a3 CHAR(3);
LET hh = MOD(tm / 100 + 11, 12) + 1;
LET mm = MOD(tm, 100) + 100;
LET am = MOD(tm / 1200, 2);
LET m3 = mm;
IF am = 0
THEN LET a3 = ' am';
ELSE LET a3 = ' pm';
END IF;
RETURN (hh || ':' || m3[2,3] || a3);
END PROCEDURE;
The Informix '[2,3]' notation is a primitive form of sub-string operator; primitive because (for reasons that still elude me) the subscripts must be literal integers (not variables, not expressions). It happens to work usefully here; in general, it is frustrating.
This stored procedure should work on any version of Informix (OnLine 5.x, SE 7.x, IDS 7.x or 9.x, 10.00, or 11.x) that you can lay hands on.
To illustrate the equivalence of (a minor variant on) the expression and the stored procedure:
SELECT begin_tm,
(MOD(begin_tm/100 + 11, 12) + 1)::VARCHAR(2) || ':' ||
SUBSTRING((MOD(begin_tm, 100) + 100)::CHAR(3) FROM 2) ||
SUBSTRING(' am pm' FROM (MOD((begin_tm/1200)::INT, 2) * 3) + 1 FOR 3),
ampm_time(begin_tm)
FROM times
ORDER BY begin_tm;
Which produces the result:
0 12:00 am 12:00 am
1 12:01 am 12:01 am
59 12:59 am 12:59 am
100 1:00 am 1:00 am
559 5:59 am 5:59 am
600 6:00 am 6:00 pm
601 6:01 am 6:01 pm
959 9:59 am 9:59 pm
1000 10:00 am 10:00 pm
1159 11:59 am 11:59 pm
1200 12:00 pm 12:00 pm
1201 12:01 pm 12:01 pm
1259 12:59 pm 12:59 pm
1300 1:00 pm 1:00 pm
2159 9:59 pm 9:59 pm
2200 10:00 pm 10:00 pm
2359 11:59 pm 11:59 pm
2400 12:00 am 12:00 am
This stored procedure can now be used multiple times in a single SELECT statement inside your ACE report without further ado.
[After comments from the original poster about not working...]
IDS 7.31 doesn't handle non-integer values passed to the MOD() function. Consequently, the divisions have to be stored in an explicit integer variable - thus:
CREATE PROCEDURE ampm_time(tm SMALLINT) RETURNING CHAR(8);
DEFINE i2 SMALLINT;
DEFINE hh SMALLINT;
DEFINE mm SMALLINT;
DEFINE am SMALLINT;
DEFINE m3 CHAR(3);
DEFINE a3 CHAR(3);
LET i2 = tm / 100;
LET hh = MOD(i2 + 11, 12) + 1;
LET mm = MOD(tm, 100) + 100;
LET i2 = tm / 1200;
LET am = MOD(i2, 2);
LET m3 = mm;
IF am = 0
THEN LET a3 = ' am';
ELSE LET a3 = ' pm';
END IF;
RETURN (hh || ':' || m3[2,3] || a3);
END PROCEDURE;
This was tested on IDS 7.31.UD8 on Solaris 10 and worked correctly. I don't understand the syntax error reported; but there is an outside chance of there being a version dependency - it is always crucial to report version numbers and platforms just in case. Notice that I'm careful to document where various things worked; that isn't an accident, nor is it just fussiness -- it is based on many years of experience.