HP OpenVMS Systems

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time limit of 10000 days?

The Question is:

Dear Wizard, In an application, we are using a unique
number to describe a manufactured product.  This number
is the number of seconds since a defined base
time (Jan 1, 1975).  As the year 2000 approaches, I began
investigating to ensure that this unique number algorithm
would still be valid.  The method we are using to
determine this number is to call SYS$GETTIM() to get the
current time in 64-bit format.  Next call SYS$BINTIM()
to convert the ASCII base time to 64-bit format.  Then we
call LIB$SUB_TIMES() to calculate the resultant delta
time ... this is the unique number.  This works great and
would continue to work except for one problem.  The
resultant delta time calculated using LIB$SUB_TIMES() must
be less than 10,000 days.  Why is this limitation there?
If the delta resultant time is an unsigned quadword
shouldn't it be able to represent 1.8 times 10 to the 19
power seconds?  Is there a graceful way to resolve this
limitation?

The Answer is:

The restriction was most likely imposed because the display form of a delta
time has a four digit number-of-days field.  You could consider using
LIB$SUBX instead.  On an Alpha, you can do the subtraction directly in the
language of your choice.