From alison Thu May 15 14:50:24 1997
Subject: Year 2000

A number of NONMEM users have asked if the year 2000 will be a problem for NONMEM.

Here is a a summary of the situation.

Years and dates appear nowhere in NONMEM or PREDPP, hence cannot cause any problems.

The year may optionally be coded in NM-TRAN data sets. If coded, then the year becomes part of the calculation of elapsed times.

If the year is coded as 4 digits, then NM-TRAN works correctly, specifically:

No values for year or date will cause an interruption,

No value for year has a special meaning,
The year 2000 is recognized as a leap year,
Day-time translation works consistently prior to, during, and after 2000.
If the year is coded as 2 digits, it is assumed to be in the 1900's.
If this is not a correct assumption, two errors may be made by the Data Preprocessor when computing relative times.

First, 1900 was not a leap year, but 2000 is a leap year. Hence, if consecutive dates in a data file are 02-28-00 and 03-01-00 (signifying February 28 and March 1, 2000), an elapsed time of 24 hours, rather than 48 hours, is computed.

Second, if consecutive dates have years 99 and 00, the computed elapsed time is negative and an error message is generated.

With NONMEM IV, there is no way to correct these errors, other than by supplying all 4 digits for the year.

With NONMEM V, there is a constant LAST20. The value of LAST20 is a 2 digit number nn that specifies the highest 2-digit year that is assumed to be in the 21st. century, i.e., to represent 20nn rather than 19nn.

A default value of LAST20 is set when NM-TRAN is installed, under the control of the installer. LAST20 may also be given any desired value at run time.

For example, with LAST20=50, then 1 and 2 digit years are interpreted as follows:

00-50 represents 2000-2050
51-99 represents 1951-1999

The elapsed time between 02-28-00 and 03-01-00 is calculated to be 48 hours, and the elapsed time calculated between the years 99 (1999) and 00 (2000) is positive.

I anticipate that this will take care of all year 2000 requirements. NONMEM V will be released well before the year 2000.

Alison Boeckmann