Computation Section
Project Management
 - Calendar

The examples to this point did not use the calendar so that the illustrations could be simplified. In practice, however, it is important to relate the project time measurement to calendar time since actual operations run in calendar time. There are two ways to construct a calendar. The easiest way is to check the Calendar checkbox on the project dialog as illustrated on the left. After the project is constructed, a calendar can be built or modified by selecting the Calendar menu item as illustrated on the right.

Calendar Checkbox
Calendar Menu Item

In either case, calendar options are entered on the Calendar Dialog. The calendar has a start date, an end date and an interval. The project start date and due date are also entered here. The project start and due dates can be changed after the calendar is constructed, but the calendar is fixed unless it is later reconstructed. The calendar range should be set large enough to encompass all possible values of the start and due dates that might later be chosen for the project. The calendar is limited to 300 entries, so the calendar interval must be chosen with this limit in mind.

The dialog assumes that the project and calendar dimensions are the same. Since we are using hours for the project, the dialog assumes that hours will be the dimension of the calendar. The contents of the field at the bottom is the number of project intervals per calendar interval. When the two dimensions are the same this number will be 1. It is possible however to chose a different project interval than calendar interval. For instance, the project interval might be "shifts". Then the bottom field would hold the number of shifts per day. This might be perhaps different than 1.


The Calendar


The add-in creates a new worksheet called PumpCal_Calendar and constructs the calendar as below. The calendar parameters are at the left and the calendar starts in column C. Column C holds indices, dates are in column D, factors relating the project to the calendar units are in E and the project units are in F. Yellow areas should not be changed. Although it is possible to change the dates in column D, most often these will also be fixed. We will see, however, that there are many instances where it is useful to change column E to indicate calendar intervals that cannot be used or are partially used by the project.

Dates on the Worksheet


The first change immediately apparent on the worksheet are the start and due dates. In most cases these govern a project rather than the start and due dates expressed in project intervals. With hours chosen as the dimension, the dates are formatted with the "month/day/year hour:minute" Excel format. Users familiar with a different date format can change these with the Excel date formatting feature.

The yellow cell (E4) is the start date offset from the calendar start expressed in the project time dimension. This is nonzero whenever the project starts later than the calendar, and it is computed by the formula in cell E4.

Further to the right on the worksheet new columns are added to express the scheduled activity start and finish times as calendar dates. When the calendar interval is day dates are computed to the nearest day. The week calendar shows dates to the nearest week.

Gantt Chart


When the Gantt chart is constructed, dates are added to the columns of the chart for every five project intervals.

Adjustments to the Calendar


The calendar can be adjusted to represent times and dates where work is impossible. For the example, we assume that the work on the pump proceeds only during the hours from 6 AM to 6 PM. The calendar is adjusted in column E by placing 0's in hours when work is not allowed and 1's in hours when work is allowed. Column F, showing project hours since the beginning of the calendar, is automatically adjusted. When looking for the date corresponding to a specified value of project hours, the Excel formulas look in column F and return the corresponding date in column D.

  On the project page it was necessary to change the due date until 2/19/05 14:00 to have keep the slack value at 2 hours. The start and stop times for the activities are adjusted to accommodate the new calendar.


The times on the Gantt chart are similarly adjusted. Note that the divisions for the Gantt chart are project hours, not calendar hours. The vertical lines every five periods separate five project hours, not five calendar hours.

  The figure below shows the calendar with the calendar interval of days and the project interval of hours. Each calendar day consists of 12 project hours. With the days interval it is easy to represent work restrictions on weekdays and holidays. The disadvantage is that all dates on the project worksheet are expressed to the nearest day. This may not be satisfactory when the project has durations measured in hours.

Usefulness of the Calendar


Actual projects are scheduled in calendar times rather than project times, and the calendar feature provides a link between the project time units and calendar dates and times. The calendar is also useful for specifying variability with respect to the calendar. In the example, some hours allow work while others do not. By changing the column holding project interval per calendar interval (col. E), variations can be described and reflected in the dates associated with the project. Other variations regard weekends, holidays and partial workdays.

While the project is on-going unexpected events can also be described on the calendar. An example would be the bad-weather days that make work on construction projects difficult. It is also possible that some days might be extended by overtime hours. Entries in the calendar that represent these events are immediately reflected on the schedule by changed completion dates. Unexpected delays would suggest to the planner that remedial action must be taken to reduce planned delays or activity times to meet the project due date.




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Operations Management / Industrial Engineering
by Paul A. Jensen
Copyright 2004 - All rights reserved