Calendars are necessary to define working programs for resources and tasks.
They can be created and modified in Project Guide, Calendar or Settings menu, Calendar.
Company calendar is the calendar used by most of the resources. It is the default calendar for every new created human resource as the Project calendar is the default calendar for project's activities (tasks).
Attention: Although the predefined calendars can be modified it is not advisable to do it.
If a resource needs a special calendar (on a regular basis, not just for some special days) select in Calendar editor a calendar with as little differences as possible to the one you need, then press New button to create a copy of it (e.g. if Standard calendar was selected, the newly created calendar name is Copy of Standard). Give it a suggestive name, as the combo box is editable. To make a day of the week as working/non-working you have to check/uncheck it. To change the working program, first step is to select desired week days (it is possible to select multiple days using Ctrl/Shift click or by mouse drag) and change them at once.
The rules to set working hours for selected days are simple:
when edit a value in From field it cannot be lower than the value in the To field of the previous row and higher than the value in the To field of the current row
when edit a value in To field it cannot be lower than the value in the From field of the current row and higher than the value in the From field of the next row
Let us take an example. Suppose you have a resource that is working part time, from 16:00 to 20:00 hours. The best solution is to create a Copy of Standard calendar and rename it the way you want. Now select all week days from Monday to Friday and let's change the working program for them. All you have to do is delete all intervals and then add any interval that you want. More exactly select the first row (8:00 to 12:00) and press Delete key. You will see how it disappears. Do the same with the remaining row (13:00 to 17:00). You will now have one single empty row that you can edit as you like. In your case enter in the From column the value 16:00 and press Enter. You will see that it will appear a new interval (16:00 to 16:01). Now all you have to do is to change 16:01 to 20:00 (double-click to edit). Press Enter and you are done.
For tasks it is necessary to make new calendars even for the slightest difference in the working time comparing to other existing calendars, as the tasks doesn't have the possibility to define its working time in Details window (this situation is far less probable to occur than for resources).
Important: changing a calendar already in use will influence the schedule accordingly!
To define a special program for certain days click on Set holidays, days off and other special days. Select the date in the calendar and modify its program or mark it as Nonworking day. To quickly get back to the normal program, as defined before at Set working days, select Week defined program. Custom program will associate to the selected day(s) a predefined program that can be changed from the Settings menu, Options, Schedule tab.
Delete the unnecessary calendars by clicking on Delete button. If you attempt to delete a calendar already in use, you'll be warned about that and invited to replace it with an existing one (select from drop down list and press Replace).