We hear this question a lot:
If I'm reading an email and want a reminder, I choose Follow-up > Custom and then click the Reminder box. The time defaults to 4:00PM - and I would like to change it to 8:00AM.
You need to change your quick click default to anything except Today and the reminder will default to the start of your workday. Right click on the flag column and choose Set Quick Click to change it.
Reminders on flags that are due "Today" (as in, set today, due today) default to have a reminder as of one hour before the end of your work day, as set in Calendar options. This allows you time to complete the task for going home; plus an 8 am reminder won't do you much good at 2 pm.
In Outlook 2007, 2010, or 2013, if you use a custom reminder for later in the day (say 11:30 AM) any messages flagged before that time will use that reminder time. Messages flagged after the reminder time has passed will not have a reminder set. The reminder dialog will default to 1 hour before the end of your workday.
Reminders for tomorrow or later use the reminder time set Task Options, or in older versions, the start of the work day (as configured in Calendar options) for the default reminder time, so you have plenty of time to work on the task before the end of the work day.
If you have the Quick Click default set to "Today" the reminder time defaults to 4 pm (or one hour before the end of the work day) or to the custom reminder set in Options, Tasks. If you use tomorrow, this week, or next week as the default quick click, the reminder defaults to the start of your work day,
Change the Quick Click flag by right clicking on the Flag icon field and choosing Click "Set Quick Click..." Select any thing but Today.
This week or Next week settings
When Quick Click is set to This Week or Next Week, the default dates are from this week or next, with the due date defaulting to the last day of your work week as set in Calendar options. For most people this will be Friday but if you configured your work week to end on a different day, Outlook will set items flagged for this week or next week to be due the last day selected (Thursday in this screenshot of Outlook 2010's dialog.)
Custom flag reminder default
The default flag and reminder time for a custom flag is based on the Quick Click setting. If Quick Click is set for Today, Custom fields will default to Today, with the reminder 1 hour before the end of the workday setting. If you change the Quick Click to tomorrow, this week, or next week, the default date will change accordingly and the reminder time will default to the start of your workday.
My Quick Click flag is set to This Week, so Custom defaults to "this Friday" and the reminder time defaults to the time I have set in Task Options, which is 8 AM. Changing the Start or End time will change the reminder date, but not the time.
Type a new task default due date
While we're on the subject of Quick Click flags and reminders, your Quick Click choice also affects the default due date for tasks entered into the To-Do's Type a New Task field.
A user asked the following question:
Can I set the default due date for tasks in Outlook 2007 to "None"? Most of the tasks I create do not require a due date. The default seems to be "Today" when I type the tasks into the blank line at the top of the To Do List but when I create the tasks using the new task button, the default due date is none
The due date is set based on the Quick Click flag choice. The To-Do Task list picks up the Quick Click flag setting. You can set the Quick Click flag to No date or use buttons or shortcuts to open task forms instead of using the Type a new task field.
Use Ctrl+Shift+K to open a new task form from anywhere in Outlook and turn off In-cell editing to hide the Type a new task field.
Reminders are set based on your task reminder settings in Task Options. (File, Options, Tasks in Outlook 2010 or Tools, Options, Task options in Outlook 2007.)