[Info-vax] date comparison format from a program

[Stephen Hoffman]

On 2019-05-20 16:58:58 +0000, gérard Calliet said:

> How doing something as simple as that in C, and not using the basic C 
> rtl (atoi,...) ,

Um, what do you think I've been grumbling about for the better part of 
a decade?

VSI is starting to address the down-revision tooling, but you're headed 
into the distant past.

The doc is pretty thin in these and other areas, too:
http://h30266.www3.hpe.com/odl/i64os/opsys/vmsos84/5763/5763profile_019.html#date_time_chap 


>  because the idea is introducing normal young people (who don't know 
> anything but C and Java) to the way something can be done with (very 
> old) VAX C and VMS run-time, and also introducting The difference 
> between C strings and string descriptors.

Descriptors?  I wrote a few articles on that topic, too. And I've been 
grumbling about the lack of integration of C and C++ with OpenVMS, and 
about the tooling, and about the archaic frameworks, and about the 
morass that are descriptors and logical names and itemlists and all the 
glue code.  It's a trip straight back to the 1980s, particularly the 
system services.  Most anybody that knows OO will probably recognize 
descriptors.  They won't like them and they'll think they're way too 
primitive.  But they'll recognize them.

Anybody that knows recent C will usually become frustrated by VAX C and 
K&R, too.  Having ported a bunch of code from K&R forward, I'd rather 
not have to port code in the other direction...


Here's a long-winded and older version of a date comparison 
else-platform (and slightly modified to document the return values), 
using Objective C:
https://stackoverflow.com/a/34992094


    NSString *dateString1 = @"Fri, 22 Jan 2016 10:46:50 +0530";
    NSString *dateString2 = @"Fri, 22 Jan 2016 10:46:50 +0530";

    NSDateFormatter *inputFormatter = [[NSDateFormatter alloc] init];

    [inputFormatter setDateFormat:@"EEE, dd MMM yyyy HH:mm:ss Z"];

    NSDate *newDate = [inputFormatter dateFromString:dateString1];
    NSDate *currentDate = [inputFormatter dateFromString:dateString2];

    NSComparisonResult result = [newDate compare:currentDate]; // 
comparing two dates

/*
NSComparisonResult can be either of these values:
 - NSOrderedAscending (-1) //if date1 < date2
 - NSOrderedSame (0)       //if date1 = date2
 - NSOrderedDescending (1) //if date1 > date2
*/



As for this case and for converting two date string to epoch and then 
comparing them, hit Stack Overflow and start reading.  For the folks 
here on macOS, Dash.app is handy as that has a complete download of 
Stack Overflow as one of its data sources.

As for C, you'll probably be using strptime here.
https://www.ibm.com/support/knowledgecenter/ssw_ibm_i_72/rtref/strpti.htm

The following shows how to get to seconds...
http://pubs.opengroup.org/onlinepubs/9699919799/functions/strptime.html

And as for all of the reasons folks get themselves in a twist when 
trying to run old versions far past what they should, I'm well aware.  
I've heard most of the discussions before.  And the folks will have 
problems, as the servers are never as isolated as the folks believe.  
Nor are the servers as reliably isolated.  And the older the stuff gets 
and the further behind it gets, the more expensive it gets.



-- 
Pure Personal Opinion | HoffmanLabs LLC