Custom Data Validation - How to call a VBA Subroutine

[jimfernish]

Overview:
Create a worksheet that allows employees to track their hours worked each day. Before allowing time entries, verify the user has entered their Name, Number and a Pay period date. After verifying the (3) fields have been filled, validate the time entry and round to the nearest 15 minute interval. Allow the user to enter;

• 8a convert this cell value to 0800, display the entry as 8:00am
• 812a convert this cell value to 0815, display the entry as 8:15am
• 12p convert this cell value to 1200, display the entry as 12:00pm
• 105p convert this cell value to 1300, display the entry as 1:00pm
• 455p convert this cell value to 1700, display the entry as 5:00pm
• 1700 complete the other validations, display the entry as 5:00pm

Goal:
After the user enters data into a cell, use Data Validation to call a Visual Basic subroutine. My thoughts are to use the ActiveCell.Value;

• Verify Name, Number & Date cells are filled
• Use ActiveCell.Value and verify time entry is a true time entry and manipulate the cell value as needed.
• If any of the (3) cells are not filled, return an error box with the missing item message.
• If the time entry is not valid, return an error box with “invalid time entry” message
• Return the updated ActiveCell.Value with the manipulated value to the active cell in the worksheet, if time is valid.

Questions:

• Is it possible to create a function that call the subroutine, validates and returns the manipulated cell contents?
• Are there any examples of the Functions & Subroutines?