How do they determine if you are a resident of uk by the number of days?

My husband is originally from uk and has just moved back after being gone for 6 years.
Do they determine residency by the number of days in UK for the tax year?

Sept 19 moved to uk
Sept 27-Nov 15th was out of country on business
In uk nov 15-22 and then went to the states
returned to uk dec 18th

So for the 2005-2006 tax period he was offically in the uk for less than 189 days is that how they count residency?

The UK tax year is 6th of April to 5th of April.

There are two considerations, there is resident and ordinarily resident.

This chart at HRMC explains if he can be considered resident or not...
http://www.hmrc.gov.uk/manuals/rgmanual/html/objects/char25.gif

This chart at HRMC explains if he can also be considered or not ordinarily resident...
http://www.hmrc.gov.uk/manuals/rgmanual/html/objects/CHAR26A.GIF

As I read it, he became resident and ordinarily resident the moment he entered the country, because I assume he intends to stay for at least 2 years as resident and 3 or more years as ordinarily resident.  Of course if he leaves and becomes resident before 3 years, then he can retrofit the determination as ordinarily resident.

Kit is correct.

Your husband is Resident in the UK from the date of arrival, providing he expected to be here for at least 2 years.  He may (or may not) be ordinarily resident if he expects to be here for at least 3 years.

He is however likely to have remained domiciled here throughout his time in the States since he was only gone for 3 years.  Consequently he will be (probably) taxable on worldwide income from 19 September (2005?) while you will only be taxable in the UK on income arising in or remitted to the UK after your date of arrival.

This may lead to some tax planning opportunities.