One trick in date/time arithmetic is learning to create and use durations.

In the bad old days of RPG, dates and times were stored as pure numbers, and we had to work pretty hard to do any arithmetic. Time values were tough: when incrementing times, we had to roll the seconds into minutes and minutes into hours when either one hit 60, and then roll the hour at 24. Figuring out the number of seconds between two times was fun as well.

Dates were even worse! Unless we were using some variation of Julian dates (which had problems of their own), just adding 1 to a date typically required a table containing the number of days for each month, not to mention a way to calculate whether you were in a leap year.

Introducing Durations

As I noted in a previous tip, RPG ILE has three data types: date, time, and timestamp (which is a combination of date and time, with precision down to the microsecond). You can perform addition and subtraction on those new types using a special syntax. A quick example:

nextWeek = today + %days(7);

How does this work? First, we have to have the right data types. We'll assume that the variables today and nextWeek are both defined as type D dates in the D-specs (or L-dates in the database). They could also be timestamps or a combination thereof, but let's keep it simple and stick with dates. The code is relatively self-documenting: add seven days to today and store the result in nextWeek.

The magic lies in the rightmost expression, %days(7). It obviously creates a variable of seven days, but what kind of variable? Is it an integer? A date? What? The answer is: none of the above. The expression %days(7) creates a temporary variable of type duration, which isn't any of the normal RPG variable types. In fact, you can't define a variable of type "duration" at all. There is no way to define variable oneWeek to do this:

oneWeek = %days(7);

nextWeek = today + oneWeek;

Odd, isn't it? But that's the way it is. You can only create a duration as the result of a call to one of the duration BIFs, and you must immediately use it in a calculation.

Using Durations

So how do these work? It's pretty intuitive, really. For dates, you can add or subtract a duration made up of days, months, or years. For times, you can use durations of hours, minutes, or seconds. For timestamps, you can use any of the above durations, as well as milliseconds. Here are a few more examples:

tomorrow = %date() + %days(1);

yesterday = %date() - %days(1);

Note the use of the current date BIF, %date(). Here's an example of a duration other than days:

dontForget = myAnniversary + %years(1);

Here are a few time examples:

anHourAgo = %time() - %hours(1);

snooze = %time() + %minutes(5);

closingTime = startWork + %hours(8) + %minutes(30);

Nice, huh? I gave myself a working day of 8 hours and 30 minutes. I'm not sure who manages to work only 8 ½ hours these days, but it does show how you can add multiple durations at once, provided they're all compatible with the view. In fact, let's take one really crazy case:

meetFriends = %date() + %days(2) + endWork + %minutes(45);

This means I'll be meeting some folks after work—specifically, 45 minutes after work two days from now. However, I can only do this if the meetFriends variable can handle both dates and times: that is, if it's a timestamp (type Z). Pretty slick, eh? Durations and dates and times together allow you to do some very sophisticated conjuring.

A Closing Caveat

You can create calculations that lead to impossible dates. For example, add one month to January 31. What is the result? Certainly, it's not February 31. So what's the answer? Well, if you use a duration of either months or years and you end up with a date whose day value is too far out for the month, RPG will set the result to the last day of that month. In my example above, you'll get either February 28 or February 29, depending on whether the year in question is a leap year. I suggest playing around with a few examples just to familiarize yourself with the results. And in fact, you may as well write a little test program, copy in my examples, and then add some of your own. Explore!