Whatever you personally commit, do everything in your power to deliver what you promised on time. Failure to do everything you could have done will result in a loss of trust.
Your supervisor has given you a project and has made December 15th a deadline for the project. Your supervisor, just like other typical supervisors, is very busy and doesn't have a lot of time to spend 1:1 with you. The supervisor is nice enough however and you have a pretty good relationship with the supervisor.
As of December 15th approaches, a project that you thought you could easily finish, as you put more work into it, you're now realizing that it's going to be nearly impossible to finish everything by that date! You start to panic. What you should do? Should you notify the supervisor now that you need more time? You know you could have worked on the project earlier but the past is past, and you can't go back in time to change this. Plus, you were working on several other projects back then and you could not have foreseen that this project would take this long, right?
In the past, the supervisor has forgotten about these deadlines. Plus, you have seen how other employees took advantage of situations like this and have asked for extensions on the project numerous times also. In addition, the supervisor has not scheduled a meeting on the 15th where you are expected to present or deliver your final project. Maybe the supervisor will let you slide this time???
You were talking to your team-mate and you promised to look into something for your team-mate and have this ready for your team-mate by next Wednesday. However, as next Wednesday approaches, you realize that you're so much busier than you've thought! Wednesday comes around and it seems like your team-mate has already forgotten about what your conversation anyway. It seems okay to let it slide this time. You will make it up to your team-mate later anyway. Two weeks have gone by and your team-mate hasn't mentioned anything and your relationship still seems quite strong.
You have an important meeting the following Tuesday. You've prepared a lot for this meeting but you still need to put in solid 20 hours of work over the weekend to be prepared for this meeting. Your supervisor as well as several colleagues of yours will be attending the meeting. Your supervisor's supervisor is also invited and will most likely attend the meeting. Your supervisor is hoping that you can impress his/her supervisor.
The weekend comes and you're ready to put in the work but things happen. Your house just got flooded and you end up spending the entire weekend, taking care of matters for the apartment. You tried to put in a few hours here and there for work, but with the flooding situation, you can't seem to find enough time. Then on Sunday, your family members get sick and have to go to the ER. You seem to be the only person who can take your family member to the hospital and take care of him/her, so you choose family first and get that taken care of.
Monday comes and you feel devastated. You're probably only about 85% prepared. Even if you put all the hours on Monday, you will only get 90% as prepared as you want. What should you do? Should you notify the team? Should you try to extend the deadline? Would that influence how your supervisor thinks about you, and the upcoming promotion you were thinking you would qualify for? Oh no!!!