Date/time data

Date/time data includes dates, such as Jan-1-2015 or 3/17/2015, and times, such as 08:25:22 AM. You can choose to display either date or time or both, such as 3/17/2015 08:25:22 AM. Elapsed time, for example, 36:33:44 (elapsed hours, minutes, and seconds), is a different type of date/time data.

How Minitab stores date/time data

Although date/time variables are displayed in a 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. For example, the date/time value January 1, 1990 6:00 AM is stored internally as the number 32874.25 because 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 December 30, 1899, 12 midnight are stored as negative numbers. For example, December 29, 1899 12 midnight is stored as -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 numeric values that correspond to a column of date/time data, choose Data > Change Data Type. In Choose type, select Automatic numeric.

How Minitab performs calculations with date/time data

Calculations are always based on the internal number that represents the date/time value. Some commands display results in the output tabs and graphs 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 may 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, 1/9/16 and 1/9/16, 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.

Tip

Use Data > Date/Time > Round Date/Time to round date/time components.

How Minitab interprets two-digit years (implicit century)

Minitab interprets 00 to 29 as 20xx, and 30 to 99 as 19xx. For example, 1/1/20 is interpreted as Jan 1 in the year 2020. 1/1/36 is interpreted as Jan 1 in the year 1936.

However, you can adjust the range of two-digit years that are 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 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 interprets this value as 12/01/2015.

When you enter a date with a missing component, Minitab handles it as follows:
• If you do not specify the day, Minitab assumes the first day of the month.
• If you do not specify the month, Minitab assumes the current month.
• If you do not specify the year, Minitab assumes the current year.

If you specify a custom date format that includes calendar year quarters but does not include other date components, 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 you specify a custom date format that includes the week but does not include other date components, Minitab assumes the current year, and the date in the specified week that corresponds to the same day of the week as January 1st of the current year. For example, if January 1st of the current year falls on a Thursday and you type 6 in a column with the date/time format display format Wkw, Minitab displays Wk6 and stores Thursday, February 5th of the current year.