Standard C Date/Time Library: Programming the World's Calendars and Clocks (Paperback)

"Standard C Date/Time Library: Programming the Worlds Calendars and Clocks" is the most definitive work on the subject yet published. At nearly 600 pages, you might think that a certain amount of overkill has been applied to Y2K and leap year computation -- this is not so. Actually, the first 100 pages take you through Y2K and all of the basic drudgery of date mechanics that so many of us programmers fail: date differences, day-of-week, leap year, and related functions. As a programmer with some years experience in medical records processing, I've written my share of these functions in the past. Latham's minimalist coding style insures that subset of C language used in his implementation is fairly the same as you would see in most other procedural langauges.

The author goes beyond mere calculation and takes us through millenia of thinking and research on how to represent this intangible yet merciless dimension known as "time". Basic concepts such as what defines a day (there are three different definitions in practice) and how the most critical yardstick - the length of a year - has been measured. The book is crammed full of such trivia as attempts to redefine the months of the year and seven day week by cultures from the French revolutionaries to communist Russia. Even the Martian year and attendant time units are given a thorough treatment. This material along with the mechanics of many other calendar systems including Hindu, Egyptian, Mayan, Jewish, and occupies somewhat more than half the book in total. If you need even one of these calendars, the book has more than paid for itself in hours of research and travel you would need to complete the task yourself.

Considerable detail is given over to calculations and historical methods of producing dates of observance. For example, the precise date of Easter as it is observed is one such problem. Other issues involve other moveable holidays which are dependent on eachother, requiring a backsolving approach. This can occur in parts of the Christian Advent calendar. Much of this is material to makers of PIMs and paper calendars (e.g. Daytimers(tm)). However, with my background in the mundane world of medical records, whether a patient was discharged on Thanksgiving or born on Christmas never entered into the calculations of fees for service. Presumably, this detail could be used in payroll calculation programs although many companies change holiday observance policies annually. For example, when Christmas falls on a Thursday, some companies will award Dec. 26th as an additional day off while others strictly don't.

My only issue of contention with this well-researched work is the titular notion of "Standard Date/Time Library". This is not a treatise on ANY existing standard, such as that covered in P.J. Plauger's "Standard C Library". In my opinion, anything which includes "Standard" in the title should have as its purpose the examination of an ANSI/ISO work as its focus. Rather, it is "Latham's Own Implementation" and not a proposed or pending standard. However, if any of the date calculation topics are of interest, I know you'll like this book.

Even if you think you already have an adequate library of date/time code in your shop, don't be too sure until you've tested it using the methods illustrated in Lance Latham's own test programs, included in the CD that accompanies the book. The Year 2000 bug is not the only one infesting date/time code!

For programmers with historical, international, or religious calendar problems to solve, this book is an invaluable reference for a wide range of past and present calendars and timekeeping systems. Lance Latham also includes a section on the forward-looking ISO 8601 standard, which is enjoying increasing usage in Europe.

I recommend this book even if you write computer programs in some language other than C. The routines in this book are easy to translate into other languages, even for programmers with only a "reading" knowledge of C. The correct usage of the routines is exhaustively documented, with all parameters, returns, and limitations spelled out for you. Whichever language you use, following Lance Latham's approach will lead to concise, fast, and reliable code.

My only caveat about this book is that the algorithms underlying the routines are not always adequately explained. Sometimes we are told no more than that an algorithm is a "standard Gregorian proleptic calendar conversion routine." Testing will verify that it does somehow give the correct results, but you may still be tempted to spend hours studying it to figure out how it works.

Not often do one see date/time routines combined with the historical background of the calendars and their problems and again, Lance reveal himself as an expert in calendars from many, many countries in his discussion of the calendars. The description of the various calendars shows his deep understanding of the problems his codes solves. Included is a CD-ROM with the entire text of SCDTL, program codes, explanations and many calendar details that are not listed in the book.

It has been a pleasure to read the book and learn from it.

Toke Nørby