Selecting Timesheets
There's two style comments, and one logic comment. The logic comment is the most significant.... you can rewrite your query as:

SELECT *
FROM `Users:Timesheets` 
WHERE 
      `start` &gt;= '{$monday}' 
  AND (`finish` IS NULL OR `finish` &lt;= '{$sunday}')
  AND `username` = '{$username}' 
  AND `shift_type` = 'ONCALL'
ORDER BY `id` ASC


Note, the style changes:


always put each SQL clause on it's own line (the FROM on a new line).
Try to indent each clause in the where clause so that the precedence of the logic is represented by the indentation.


Update: I assume, based on the variable syntax, and so on, that this SQL is not a direct query, but is rather the input to some string substitution that expands the {$...} variables. This is potentially vulnerable to SQL Injection attacks. Are you sure you shoud be running your SQL this way? Have you considered prepared statements? Is this a prepared statement?