Using Duration Aliases and Duration Options

This chapter provides overviews of duration aliases and duration options and discusses how to:

• Define a duration alias.

• Select and set up a calculation method.

Understanding Duration Aliases

The system calculates a duration by subtracting an earlier date from a later date. The remainder is the duration.

A duration alias measures the length of a period of time. The most common durations are ages. An employee’s age on a particular date is the duration between that date and the birth date. Another common duration is elapsed time service, the period of time between two service-related events, such as a hire and a termination.

Setting up a duration alias is a two-part process. You need to:

• Specify the endpoints for the duration—for example, birth date and event date.

• Select a calculation method.

You define calculation methods on the Calculation Options and Conversion Options pages.

Duration aliases always produce decimal results. For example, a duration of six years and three months is expressed as 6.25 years. When you create a duration option you establish a method for converting raw dates—formatted in years, months, and days—into decimal equivalents.

The decimal that represents the total duration is the primary result of a duration alias. However, duration aliases also manipulate the decimal value to derive six other secondary results.

See Also

Setting Up a Raw Date

Understanding Duration Options

This section discusses:

• Duration options

• Date math methods

Overview of Duration Options

Duration options instruct the system on the details of calculating a duration. You set up duration options separately from duration aliases; this enables you to easily reuse a duration option.

Date Math Methods

There are three methods of subtracting dates:

• Decimal date subtraction

• Raw date subtraction

• Period counting

The initial results of raw date subtraction and period counting are in year-month-day format. The system converts this duration to a decimal equivalent after performing the subtraction.

Decimal Date Subtraction

With decimal date subtraction, the date endpoints are converted to decimal values before the subtraction; the result is therefore a decimal.

Raw Date Subtraction

Raw date subtraction subtracts days from days, months from months, and years from years. The raw dates are formatted as yyyy/mm/dd. The following example illustrates this method:

  1995/12/11
- 1988/09/05
   __________
   0007/03/06 or seven years, three months, and six days.

Consider, however, the following subtraction:

  1990/11/06
- 1977/12/13

You cannot subtract 13 days from 6, so you have to convert one of the months to days. But when you change the 11 month to 10, how many days do you carry to the days column? When you set up raw date subtraction methods, you choose whether to use the actual number of days per month or an assumed number days per month.

If you use the actual number of days per month, the number of days depends on the month in question. In this case, the tenth month (October) is the most recent full month that you can convert to days. October has 31 days, so 1990/11/06 becomes 1990/10/37. You still have to convert one year to months to complete the subtraction, but you know there are always 12 months per year. You end up, then, with a date of 1989/22/37 and your equation yields:

  1989/22/37
- 1977/12/13
   __________
    0012/10/24 or 12 years, 10 months, and 24 days.

If you assume 30 days per month, 1990/11/06 becomes 1990/10/36, and after you again convert 1 year to 12 months, you end up with 1989/22/36. Your equation is now:

  1989/22/36
 - 1977/12/13
   __________
   0012/10/23 or 12 years, 10 months, and 23 days.

Period Counting

Period counting measures a duration by starting with the later date, then subtracting defined periods (years, months, and days) until reaching the earlier date. You choose the period or periods to count.

You will likely use period counting durations to measure the difference between the normal and actual retirement dates. You use this information to determine early retirement penalties. For example, if your plan’s early retirement reduction is 3 percent per year, you use period counting to determine the number of years between the normal and actual retirement dates.

You’re also likely to use period counting durations for measuring elapsed time service or PRSA coverage periods.

Let’s calculate the duration between 1977/12/13 and 1990/11/06. (These are the same dates used for the second example under “Raw Date Subtraction.”) You count years, months, and days. Period counting methods always start with the later date and count backwards.

First you count years, and you find 12 complete years in the interval. This takes you from 1990/11/06 back to 1978/11/06. When you count months, you find 10 complete months in the interval. This brings you to 1978/01/06. When you count the days, you always use actual days per month. Your last count includes 6 days in January and 18 days in December, for a total of 24 days. The result is 12 years, 10 months, and 24 days.

Although it appears that period counting always gives the same result as raw date subtraction, there is a difference. Period counting counts actual days for the first full month between the two dates, whereas raw date subtraction counts actual days for the last full month between the two dates.

For example, consider the duration from February 15 to August 10 of the same year. If you use period counting, you end up with 5 full months from August 10 back to March 10, plus 23 additional days between February 15 and March 10. If you use raw date subtraction, you end up with 5 full months between February 15 and July 15, plus 26 days between July 15 and August 10. The difference occurs because February only has 28 days, whereas July has 31 days.

Defining a Duration Alias

To set up a duration alias, use the Duration Alias (DURATION) component.

This section lists the page used to set up a duration alias and explains how to set up a duration alias.

Page Used to Set Up a Duration Alias

Setting Up a Duration Alias

Access the Duration Alias page.

Date-To and Date From	Enter the boundary dates for the period you want to measure. Either date can be a constant date or an alias. Of course, if the two dates are both constant, the duration would be constant. Therefore, at least one of your date endpoints is normally an alias.
Duration Option	Enter a duration option to establish a calculation method.

Selecting and Setting Up a Calculation Method

To set up a calculation method, use the Duration (DURATION_OPTIONS) component.

This section lists the pages used to select and set up a calculation method and discusses how to:

• Select calculation options.

• Set up a raw date.

Pages Used to Select and Set Up a Calculation Method

Selecting Calculation Options

Access the Calculation Options page.

Date Math Method

Select one of the following date math methods:

• Raw Date Subtraction

  Also select options in the Raw Date Subtraction, Second Period/Month Round Rule, and Third Period/Day Round Rule group boxes.

• Period Counting

  Also select options in the Period Counting Subtraction, Second Period/Month Round Rule, and Third Period/Day Round Rule group boxes.

• Decimal Date Subtraction

• User Code

Raw Date Subtraction

Convert months to days in order to perform subtractions when the earlier date has fewer days than the later date. For example:

  1990/11/06
- 1977/12/13

Period Counting

If you choose the period counting method, you need to indicate the period or periods to count. You can count either one type of period, such as years, or you can count successively shorter periods, such as years and then months.

You can define up to three types of counting periods: years, months, and days. You can use any combination of these periods, but you must use them in order: from longest to shortest.

Although you may not want to express shorter units in the final count, you may still want to count them for rounding purposes. For example, if you are only going to count years, but you are going to round by months, you also need to count months.

Second Period/Month Round Rule and Third Period/Day Round Rule

If you use raw date subtraction or period counting (both based on raw dates), you can choose to round the dates before calculating a duration. You specify the rounding options in the Second Period/Month Round Rule and Third Period/Day Round Rule group boxes.

You use these group boxes as follows:

• For raw date subtraction, you specify the month round rule in the Second Period/Month Round Rule group box and the day round rule in the Third Period/Day Round Rule group box.

• For period counting, you specify the second period in the Second Period/Month Round Rule group box and the third period in the Third Period/Day Round Rule group box.

For example, to round days to months when using raw date subtraction, use the day round rule. To round days to months when using period counting, first check which period counts days. If the second period counts days, use the second period rounding rule, even though the label is Month Round Rule.

Use the options in these group boxes as follows:

• To round up only, select Round Up if Periods > and enter the threshold. For example, you could round days up to a full month after 25 days.

• To round up or down, choose Round Up if Periods > … Else Round Down and enter the number of periods.

• If you do not want to round at all, select None.

Note. These options round the dates that are the duration endpoints. That is, the dates are rounded before the system performs raw date subtraction or period counting. To round the duration itself, or to round the endpoints before performing decimal date subtraction, use the Month Conversion Rules and Year Conversion Rules on the Conversion Options page.

Setting Up a Raw Date

Access the Conversion Options page.

Unless your source dates are decimal dates, the system converts the raw dates or durations to decimal dates. If you use raw date subtraction or period counting, the system converts the duration after performing the date math. If you use decimal date subtraction, the system converts the endpoints before performing the subtraction.

When you convert raw dates and durations to decimal equivalents, you have to convert months and days to partial years. There are a number of ways to do this. Specify your conversion options in the Decimal Conversion Assumptions and Decimal Conversion Method group boxes.

Decimal Conversion Assumptions

Specify your assumptions about the number of days in a month and in a year. The system uses these values to convert dates to decimal values.

Decimal Conversion Method

Select one of the following options to indicate how to convert dates into decimal values:

Using Decimal Conversion Methods

This section provides examples of the decimal conversion methods. These examples convert the “raw” duration five years, six months and six days to a decimal value. The process is similar to that of converting a raw date to a decimal before performing decimal date subtraction.

For the examples, assume 30 days per month, 365 days per year, and four decimal places.

1. Years + { [ Months * Days per Month ] + Days } / (Days per Year)

   Years
      + (months converted to days + days) converted to partial years
   Years = 5
   Months converted to days = 6 * 30 = 180
   Days = 6
   Total days = (180 + 6) = 186 
   Total Days converted to partial years = 186 / 365 = 0.5096
   Result: 5.5096

2. Years + [ Months / 12 ] + [ Days/ Days per Year]

   Years 
     + months converted to partial years
      + days converted to partial years
   Years = 5
   Months converted to partial years  = 6 / 12 = .5000
   Days converted to partial years = 6 / 365 = .0164
   Result: 5.5164

3. Date Tables: Years + (Actual Days)

   To find the actual days value of six months and six days, you need to know which six months and six days they are. Assume this portion of the duration starts on January 1, 1992. This means the months are January through June, and the six days are then the first six days of July. Add 31 (January), 29 (February in a leap year), 31 (March), 30 (April), 31 (May), and 30 (June), and 6 (the remaining days) for a final value of 188. This is converted to years using the actual number of days in this year: 366 because of the leap year.

   Years
      + actual days converted to partial years [always using 365 days/year]
   Years = 5
   Actual days converted to partial years = 188 / 366 = .5137
   Result: 5.5137

Month Conversion Rules and Year Conversion Rules

If you use decimal date subtraction, the system uses the Month Conversion Rules and Year Conversion Rules settings to round the duration endpoints (which are decimal values) before the subtraction that determines the duration. You cannot further round the final result.

If you use raw date subtraction or period counting, the system uses the Month Conversion Rules and Year Conversion Rules settings to round the final duration result.

The settings work the same regardless of which value you’re rounding. Select Up to round to the next month or year, Down to round to the previous month or year, or Near to round to the closer of the two rounded values. Select None to avoid rounding altogether.

Note. For decimal date subtraction, these rounding options operate on the decimal date endpoints before the system calculates the duration. For raw date subtraction or period counting, these rounding options operate on the duration itself. To round the endpoints for raw date or period counting methods, use the rounding rules on the Calculation Options page. You cannot round a duration produced by decimal date subtraction.

Using Monthly Equivalents

A duration produces a primary result, a decimal value representing the entire duration, and six secondary results (called results 2-7). The secondary results are:

In this table:

• Results 2, 4, and 6 deal in whole months.

  This means that any remaining days are rounded.

• Results 3, 5, and 7 do not necessarily round days.

  If you select Monthly Equiv Flag on the Conversion Options page, these values are rounded according to the settings in the Month Conversion Rules and Year Conversion Rules group boxes on that page.

• The Month Conversion Rules settings apply to the results that round days.

  This is all three results if you use monthly equivalents.

• You can round months and years up, down, or near, or you can choose not to round.

Note. If you do not use the secondary duration results, disregard the Monthly Equiv Flag field.

The following table provides examples of the seven results of a duration calculation. It shows the effect of selecting Monthly Equiv Flag. The examples assume that both the monthly and yearly rounding options are set to Near and that there are 365 days per year.

To reference the secondary results of a duration, add the appropriate numeric suffix to the duration name. These numbers correspond to the result numbers shown in the previous table—for example, AGE2 references result 2 for AGE.