[Info-vax] date comparison format from a program

[Stephen Hoffman]

On 2019-05-20 13:27:49 +0000, gérard Calliet said:

> Is there a way to do from a compiled program (in C, for example) 
> something as simple as done by the lexical f$cvtime with "comparison" 
> parameter in DCL to get date strings which can easyly sorted? Pure VMS 
> realm whished.
> 
> (I have to write program examples in C with VMS dates format and 
> associated routines and compare with RtlC usages).

If you really want to use text, use a text format based on ISO-8601.

For C, the standard C timekeeping API works correctly and allows for 
comparisons (e.g. difftime), and there are routines available in the 
dtss library that allow converting to (e.g. 
utc_vmslocaltime,utc_vmslocaltime) and from (e.g. utc_mkvmslocaltime, 
utc_mkvmsgmtime) OpenVMS native and epoch, and comparing (e.g, 
utc_cmpintervaltime).
http://h30266.www3.hpe.com/odl/i64os/opsys/vmsos84/4493/4493pro_016.html#port_api 

http://h30266.www3.hpe.com/odl/i64os/opsys/vmsos84/5763/5763profile_019.html#date_time_chap 


if you must use RTL and system services, then use the "newer" UTC 
system service and not the older $numtim and related calls.
http://h30266.www3.hpe.com/odl/axpos/opsys/vmsos84/5841/5841pro_075.html#sys_time_operations 


Convert to UTC, and compare.  Or use a C or dtss API that is timezone-aware.

For system services source code examples from C, see:
http://www.eight-cubed.com/examples.shtml

I'd expect to be able to get wrong answers with the older and 
should-have-been-deprecated OpenVMS system service APIs including 
$numtim, as they're not timezone and not DST-aware.  The UTC system 
services do better here.

-- 
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