Working with different calendars

First of all, do not simply edit the Standard Calendar.
The Standard Calendar is 5 days/week and 8 hours per day.
If you change it you won’t have a 5 x 8 Calendar anymore.

Use Tools, Change Working Time, and copy the Standard Calendar. Give it a distinctive name such as "AA 5D 7.5H".
Edit the working times, save.

Do the same thing again but name it "AA 5D 8H".
This one doesn’t have to be edited because it is identical to the Standard Calendar.

You can then choose either calendar as the Project Calendar in Project, Project Information.
All Tasks use this unless you give them a different one.
You will also have to select the calendar as the Base Calendar for the Resources.

Thanks a lot for your reply.

However, in my version of MS Project (2007 standard) I do not have the option to edit working times. Any ideas why this is?

The working times to the right of the calendar is grey and not possible to edit.

Christian,
unfortunately, it is not as simple as that:
a. have a look at Calendar Options (Tools, Options, Calendar) and consider that you have only ONE number of hours per day for each project, which means that the tasks that use the 7.5H/D calendar will be longer that one complete day, say a task starting Monday 8:00 AM will finish Tuesday 8:30 AM instead of Monday 4:30 PM; you will have to enter these tasks duration in hours instead of days to avoid mistakes
b. do you want to assign this new calendar to tasks or resources? if to tasks, open the Task Information dialog, select the Advanced tab and select the calendar in the drop-down list; if to resources, display the resource sheet, go to the Base calendar column and select the new calendar
To change the start and finish time of the day in MSP2K7:
a. create a new calendar as described before,
b. select this calendar in Tools, Change Working Time,
c. click the Working Week tab at the bottom of the dialog box, then give the week a name, such as "7.5H/D week" ("Default" by default) and a start and finish dates,
c. click Details and give each necessary day a start and finish time; I guess that you will enter 8-12 -- 13-16:30 on Friday
Click OK, OK to close all
Then, you certainly want to use this calendar in more than ONE project only? Go to Tools, Organize, click the Calendars tab, copy the new calendar from your current project in the GLOBAL.MPT file (the MSP library).
That’s all, chap
Alexandre