To manage development time, maintain a concise and up-to-date project plan. A project plan is an estimate, a schedule, a set of milestones for marking progress, and an assignment of your team or your own time to each task on the estimate. It should also include other things you have to remember to do, such as meeting with the quality assurance people, preparing documentation, or ordering equipment. If you are on a team, the project plan should be a consensual agreement, both at the start and as you go.
The project plan exists to help make decisions, not to show how organized you are. If the project plan is either too long or not up-to-date, it will be useless for making decisions. In reality, these decisions are about individual persons. The plan and your judgement let you decide if you should shift tasks from one person to another. The milestones mark your progress. If you use a fancy project planning tool, do not be seduced into creating a Big Design Up Front (BDUF) for the project, but use it to maintain concision and up-to-dateness.
If you miss a milestone, you should take immediate action such as informing your boss that the scheduled completion of that project has slipped by that amount. The estimate and schedule could never have been perfect to begin with; this creates the illusion that you might be able to make up the days you missed in the latter part of the project. You might. But it is just as likely that you have underestimated that part as that you have overestimated it. Therefore the scheduled completion of the project has already slipped, whether you like it or not.
Make sure your plan includes time for: internal team meetings, demos, documentation, scheduled periodic activities, integration testing, dealing with outsiders, sickness, vacations, maintenance of existing products, and maintenance of the development environment. The project plan can serve as a way to give outsiders or your boss a view into what you or your team is doing. For this reason it should be short and up-to-date.