Log work default time

[hellznrg]

When I log work on an issue, the default start date/time is always set as the current time. However this time always has to be changed. 9 times out of 10, the date/time has to be set to (current time) - (amount of time spent on the task). It would be a huge usability improvement if when the "time spent" field loses focus, the date/time field is automatically set to (current time) - (amount of time spent on the task).

For example: suppose the current time is 20:40.

When I click on "Log work", the form comes up with the current time (20:40). Then I type in "5 h" as the amount of time spent. When I exit that field, the "date started" field should automatically recalculate and show 15:40.