For Work logs you can choose from two different "date/time"-schemes. You configure which of these you would like to use in this page.
* Use only work date and time spent
Choose this if you only want to specify a "Work date" and "Time spent", e.g. Date: 2012-06-25, Time spent; 4h
* Use start and end date/time
Choose this if you need to have know exactly at which time the work started and ended, e.g.: "Start date/time": 2012-06-25 10:00, "End date/time": 2012-06-25 12:30.
When you use this option the Time spent field will be automatically calculated and if you have a Start date/time and specify Time spent the "End date/time" will be calculated instead.