There are two major different types of numbers that are stored in worksheets: standard sequential values and non-standard sequential values. Standard sequential values are defined by spreadsheet programs as starting at a certain date and having a certain increment.

A non-standard numeric date is similar to the standard numeric dates, but in these situations, the starting value and increment is determined by the user. So, the value 1 can be any date/time. The increment can be any time interval. So, the value 2 can refer to the next day, like standard serial date numbers; or it can refer to the next second, next year, or a different increment entirely. Some recording devices will record in this manner. This requires additional steps in Grapher to display the dates.

Let's assume that the value 1 is January 1, 2012. Let's further assume that the data should be collected every day. So, the value 2 is January 2, 2012; the value 3 is January 3, 2012; and so on. Our data set now looks as follows:

The dates in the left column are in custom numeric format. Grapher will
set the axis to numeric when creating the graph.

To create a graph from this data, select the two columns and click the Home | New Graph | Basic | Line Plot command. The X axis will be in numeric format.

The X axis is automatically displayed with numbers instead of dates.

After the graph is created, the axis can be converted to dates. To convert the numbers to dates:

1. Click on the X Axis 1 in the Object Manager to select it.
2. In the Property Manager, click on the Labels tab.
3. In the General section, verify that Use date/time format is unchecked.
4. Set the Label source to Numeric date/time.
5. Click the Set button next to Date/time settings.
6. In the Date/Time Labels dialog:
   1. Set the Axis value to 1.
   2. Set the Date to 1/1/2012.
   3. Set the Time to 12:00:00 AM.
   4. Set the Axis increment to 1. 
   5. Set the Corresponding time increment to Day.
   6. Click the Set date/time format button and set the Language (Country) and Predefined date/time formats to the desired format, such as English (Unites States) - [$409] and MMM-dd. If a predefined format is not listed, you can type the desired format in the Date/Time fomat (edit to change) box. Click OK.
   7. Click OK.

The axis converts the numbers to dates using the non-standard numbering scheme defined in the data. The axis tick label format must be set in the Format option in the Date/Time Labels dialog. The format in the Label Settings section of the Property Manager will not have an effect on the axis because the axis is using the non-standard numbers, not date/time formatting.

The X axis is changed to display with date/time values.

The axis tick mark spacing can be set on the Ticks tab of the Property Manager, using the numeric spacing, not the date/time spacing. To change the tick mark spacing:

1. Click on the X Axis 1 in the Object Manager to select it.
2. In the Property Manager, click on the Ticks tab.
3. Highlight the value next to Spacing and type the desired value. A week in this situation is equal to 7. So, type 7 and press ENTER on the keyboard to make the Spacing change. Then change the Divisions in the Minor Ticks section to 7, so one minor tick mark is 1 day. 

Updated July 2019