notice time required by new law, advice

http://www.globalhrlaw.com/resources/employment-law-review-2014-belgium

During the first five years of service, the notice period develops progressively: Quarterly during the first two years and then annually. The period of notice shall be:

First quarter = 2 weeks
Second quarter = 4 weeks
Third quarter = 6 weeks
Fourth quarter = 7 weeks
Fifth quarter = 8 weeks
Sixth quarter = 9 weeks
Seventh quarter = 10 weeks
Eighth quarter = 11 weeks

You and your employer are bound by the law in force on the date that you signed the contract.
Whatever changes have been enacted since then are irrelevant unless those changes were specifically stated to be applicable to all existing contracts. It is extremely unlikely that such was the case and, even if it was, it is up to your employer to prove it rather than just saying that it is so.
Give one week's notice in writing quoting the contract which you signed and let him PROVE that you are wrong.

As an aside; many of us gave 3 months rent when we signed our rental contract. That law has now been changed and the required deposit is only 2 months; no-one who had given 3 months got a month refunded when the law changed!

I think your employer is wrong. I think he is applying the following rules which as far as I can tell apply once the probation period is ended:

An unofficial translation of the law that I have says:

TERMINATION OF A CONTRACT FOR AN INDEFINITE PERIOD
The notice periods which are notified as from 1 January 2014 take effect on the Monday following the day on which the letter of notice is deemed to have been received.
They are fixed as follows, without prejudice to what is in particular stipulated in articles 37/5, 37/6, 37/7 and 37/11 of the Employment Contract Act of 3 July 1978.
The employment contract for white-collar worker concluded for an indefinite period, the performance of which began prior to 1 January 2014 can be terminated provided that the notice periods stipulated in article 67 to 69 of the Act of 26 December 2013 with regard to the notice periods and the waiting day and accompanying measures are observed (see below), without prejudice to the departure provided by article 73 of the present Act of 26 December 2013.
These periods are determined by adding up two periods, more particularly:
1. the period calculated in accordance with the continuous length of service acquired on 31 December 2013 and determined as follows:

Annual wage on 31 December 2013: €32.254 or less, length of service 0 to 5 years = employer 3 months per starting period of 5 years of service, worker 1.5 months.

Annual wage on 31 December 2013: €32.255 - €64.508, length of service 0 to 5 years = employer 1 month per starting period of 1 year of service with a minimum of 3 months, worker 1.5 months.

Annual wage on 31 December 2013: more than €64.508, length of service 0 to 5 years = employer 1 month per starting period of 1 year of service with a minimum of 3 months, worker 1.5 months (i.e. same as above but the period for the worker is different for longer employment terms).

2. the period calculated in accordance with the continuous length of service acquired as from 1 January 2014 and determined to the rules set out in point D (a) [which concerns indefinite contracts that started as from 1 January 2014] i.e.:
0 to <3 months: employer 2 weeks, employee 1 week
3 months to < 6 months: 4 weeks, 2 weeks
6 months to < 9 months: 6 weeks, 3 weeks

Which is not particularly clear. However, I think for you it (would) mean you had 3 months employment prior to 1 January 2014 and 7 months employment after 1 January 2014. So, 1.5 months plus 3 weeks. In other words, 9 weeks as he has told you (though perhaps not in the way he explains).

HOWEVER...the amendment also says:
The employment contract, the performance of which began prior to 1 January 2014 can be terminated during the probation period provided that the notice periods laid down by articles 48, 79 and 81 of the Employment Contract Act of 3 July 1978 as in force on 31 December 2013 are observed.

- White collar worker: 1 month > end of probation - notice by employer=7 calendar days, notice by worker = 7 calendar days.

I think this (i.e. the "still within probation") is what has changed from the old law - although in your case it seems the notice period was in any case 1 week.

Hope this helps.

"You and your employer are bound by the law in force on the date that you signed the contract."

Yes, end of. You are still on 12-month trial period and can resign at a weeks notice.

Put the letter in, walk away after a week, and if the silly twat wants to argue about it in court, let him waste his money.

Focus on the new job.

What they ^^ said

"there was a trial period of 12 months stated in the contract. During this time the notice period was agreed to be 1 week from both sides"

And that still holds.

as "J" notes above, just hand in your notice and leave after a week.

"just hand in your notice and leave after a week."

Even if you are wrong,
1. It is not worth compromising the new job
2. Legal action on their behalf is going to be expensive, messy and very time and resource consuming
3. I doubt they're that vindictive
4. Do it just after you get paid if you're in any doubt and the lack of pay might be an issue.

You only need one week as you are still in the trial period. Make sure you send your resignation letter by registered mail asap.

My colleague was fired with 30 minutes noticed after more than a decade of service. Nice. And I heard so many similar cases in the past 5 years

If they didn't need you, you would be out of the company in less than the stated week mentioned in your contract. Companies are merciless. So send your resignation letter, say your goodbyes and land somewhere better.

they had the upper hand in the last few years but I hope the tide is turning.

According to the new legislation the trial period can no longer exist in Belgium for employment contracts started on 01 January 2014 or after.

The good news for the original poster is that there is an exception clause, which retains that for contracts which has started before 01 January 2014 and include defined trial period, the original agreement holds until the end of the trial period. You'll find the whole text in here:
http://www.emploi.belgique.be/defaultTab.aspx?id=40420

That said, it is good to notice that the exception does not concern calculation of the notice periods in case the original trial period has ended. For contracts that have started before beginning of 2014, the notice period is calculated in two parts. First you need to calculate what would have been your notice period on 31 December 2013 as per the old legislation. Then you add to that the notice period accrued as per the new legislation as if you were hired on 01 January 2014. More info in here:
http://www.emploi.belgique.be/defaultTab.aspx?id=40415

The notice periods as per the new regulations grow gradually as noted in the first message of MEHA, However the periods in that message are notice periods in case employer finishes the contract. The notice period for employee resigning is half of the notice period the employer has to follow in case of dismissal.