While at my current company (Old Company) I accepted a job offer as I was leaving on a three-week holiday. I told the new company (New Company) I would resign the day I returned to work, and complete three-weeks of notice period. (I considered resigning just before or during vacation to be unprofessional as my notice period would not be effective to my employer.)
Because of my five years at Old Company, I knew the standard notice period is 6 weeks (national labor law) with the option to complete less of the notice period by paying half a days wage for each unfulfilled day. My intention was to complete what I saw as a reasonable three weeks (given my current commitments), and pay the rest.
Now I have handed in my resignation and my manager wishes that I complete the full six-weeks in order to finish a project (of roughly 10 months duration) that is somewhat near completion*. My current role is that of a project leader for a ~10 person team.
From New Company's perspective, they are giving me the standard six-weeks (3 weeks vacation + 3 weeks notice) for notice period at Old Company.
From Old Company's perspective, I'm only completing half of their normal notice period (due to the holiday).
To try to remedy the situation I have talked to New Company, politely requesting whether they can wait three more weeks for me, as I have an important project to finish. Their answer was that they really wish me to start on the agreed date because they have a client waiting, etc.
My thoughts are that either a) my proffered three-weeks notice to Old Company is sufficient and I'm compensating for the unfulfilled days, b) I screwed up by offering to start at New Company, completing only half on the standard notice period.
The trouble with possibility (a) is that as my current manager will know Old Company's commitments better than I, only his opinion on how much notice is sufficient counts. The consequence of not completing full notice is a hurt relationship with my manager going forward (and harder to get a referral in the future from him). I have tried other ways to improve the situation, like offering to help my manager after starting at the new company, after work hours or on weekends.
Possibility b) seems more likely because I wasn't in a strong position to make the assumption that anything less than the standard notice period suffices. Remedying this would involve the difficult question of asking New Company for more time to complete the full notice period prior to joining their company. This would allow Old Company a better chance of finishing the project on-time and would also allow me to maintain the excellent relationship I have with my manager. The danger here is that they rescind their offer due to their client needing me to start on the given date, or else need to find alternatives.
I have also read here about how a serious company should do all it can to allow their future employees to transition from their old company.
Is my reading of the situation correct and is the best way forward renegotiating my notice period with my future employer? What alternatives come to mind?
*: Deadlines have slipped multiple times on this project - currently my manager thinks the project will be finished by the time I complete a six-week notice, but I think that is an optimistic view and that the project will need at least 2 or 3 months to complete.