Tier 5 Stamp date or 'valid until'?

Hello - I was advised to apply for my work visa before booking flights etc and told my visa would be valid FROM THE STAMP DATE when i arrived in the UK ( provided it was within 90 days / 3 months of my visa 'valid from' date). It is EASILY within 90 days yet returning from a holiday recently the immigration official who checked my passport told me I wasn't allowed to work past my 'valid until' date (20/06/14) even though the 2 year work visa was stamped on september 9th 2012 - therefore I was under the impression I could work until September 9th 2014 (2 years from stamp date)
Please tell me the immigration official at the gate was over tired and incorrect?
thanking you in advance -

You can work/stay until the valid until date on your visa. All work visas are finite and the end date on the visa is what you have to go by, not 2 years from the date you entered.

I have a Tier 5 GAE visa that ends mid-January 2016 (15 Jan.).

I am thinking now about booking my one-way travel back to the US.

My understanding was that I could stay on for a week or so AFTER the 15 January date, just to take care of personal things, such as cleaning my flat, closing banking accounts etc., as long as I no longer did any actual work.

Is this true -- or do I really need to leave the country by 15 January? I imagine that there is a good chance that nobody would come in that one week to deport me, but I would not want to jeopardize any future visits to the UK.

Thanks for any advice

You must leave by the 15th or you will be in breach of your landing conditions.

"By the 15th" or ON the 15th?

In other words, would I be in violation if my flight leaves on the 15th?

You'll be fine on the 15th.

Thanks for the info!