Can somebody recommend a simple app to do the following: keep a list of tasks and estimations of time required to complete them. When I insert an item between e

Can somebody recommend a simple app to do the following: keep a list of tasks and estimations of time required to complete them. When I insert an item between existing tasks, all the dates of subsequent tasks should be automatically updated. That is really all I need, but so far I have not find to right tool for this.