Time is a valuable resource. All of us have the same amount of time irrespective of our position in life. None of us are denied of this resource at any point of time. We do not have the power to stop its constant and continues flow. If you have a flex time, the work will occupy the entire time available. If you set to finish a job in 30 minutes you can finish the same in 30 minutes. If you set one hour for the job completion, you will take one hour for the same work. This means that the job content being the same, the completion time depends upon the duration you fix.
You have to fix a time frame for every activity. Time frame means fixing a start point and end point for a job. For example, before we start a meeting, we should fix the time when the meeting will end. Normally circulars mention the start time of a meeting and not about the end time.
Whenever you have to start a job, instead of considering how much time is unavailable, consider how much time you want to take to complete the job. Instead of merely talking about the contents of the job, build a buffer time and have a realistic time estimate. Anticipate what problems you are like to encounter and how you can overcome the same. Anticipate the resources and skill you may require for completing the job within the time fixed. Buy a timer. You can also have other systems to remind you about the deadline such as alarm clock.