What is date/time data?

Date/time data includes dates, such as Jan-1-2015 or 3/17/2015; times, such as 08:25:22 AM; or both, such as 3/17/2015 08:25:22 AM. Date/time data also includes elapsed time, such as 36:33:44 (elapsed hours, minutes, and seconds). Minitab stores dates and times as numbers, but displays them in whatever format you choose.

Tip

If you import date/time data into Minitab and determine that Minitab did not recognize it as date/time, you can change the data type to date/time. Choose Data > Change Data Type, select the column, then select Date, Time, or Elapsed time as the type.

How Minitab stores date/time data

Although date/time variables are displayed in date/time format in the worksheet, they are stored internally as numbers. The internal number includes an integer and a fraction. The integer represents the number of days since December 30, 1899; the fraction represents the fraction of the day completed. For example, the date/time value January 1, 1990 6:00 AM is stored internally as the number 32874.25 since there are 32874 days since December 30, 1899, and 6:00 AM is one quarter of a day.

December 30, 1899, 12 midnight is stored as 0. Dates and times before that date and time are negative numbers. For example, December 29, 1899 12 midnight is -1. Valid date/time values range from January 1, 1000 A.D. to December 31, 4000, with leap years determined by the proleptic Gregorian calendar.

Tip

To see the representative numeric values of a column of date/time data, choose Data > Change Data Type, select the date/time column, then select Automatic numeric as the type.

How Minitab performs calculations with date/time data

Calculations are always based on the internal number of the date/time variable. Some commands will display results in the Session and Graph windows as a number, and not as a formatted date/time value. Date/time values that are stored in constants are displayed as numbers.

Some commands will give you unexpected results when using date/time data. For example, consider an analysis of variance with a date/time variable as a factor. Two values in the date column, 1/1/99 and 1/1/99, may appear in the worksheet to be the same value because the column is formatted to display just the date portion. But the two values may have different time portions (for example, 10:30 PM and 8:20 AM) and, therefore, have different internal numbers. The analysis of variance will treat each value as a different factor level.

Note

Minitab dialog boxes accept date/time data only in their default formats. These defaults can change depending on the regional settings of the Windows Control Panel.

How Minitab interprets years represented by two digits (implicit century)

Minitab interprets years represented by two digits as follows:
  • 00 to 29 as 20xx (for example, 1/1/20 is interpreted as Jan 1 in the year 2020)
  • 30 to 99 as 19xx (for example, 1/1/36 is interpreted as Jan 1 in the year 1936)

However, you can adjust the range of two-digit years assigned to each century by customizing the calendar option in the regional settings of the Windows Control Panel. When you restart Minitab, it automatically updates its default date/time settings with the change.

How Minitab handles missing date/time components

Minitab always internally stores a complete date which includes a month, day, and year component. When you enter a date that does not include one of these components, Minitab stores a placeholder value for the missing component. For example, suppose the display format for a column is mm/yyyy. If you enter 12/2015, Minitab stores this values as December 1, 2015.

When you enter a date in a column without specifying a day, month, or year, Minitab assumes the following:
  • If the day is not specified, Minitab assumes the first day of the month.
  • If the month is not specified, Minitab assumes the current month.
  • If the year is not specified, Minitab assumes the current year.
When you enter a quarter or week value in a column without specifying a day, month, or year, Minitab assumes the following:
  • If the format is Qq, Minitab assumes the first day of the quarter, the first month in the quarter, and the current year. For example, when you type 1 in a column with the date/time display format Qq, Minitab displays Q1 and stores January 1st of the current year.
  • If the format is Wkw, Minitab assumes the first day of the specified week and the current year. For example, when you type 1 in a column with the date/time format display format Wkw, Minitab displays Wk1 and stores January 1st of the current year.
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