When do the 12 weeks start..

[peonylane]

On 15th March as per government advice I self isolated due to being in the high risk category.
My letter arrived on 23rd advising me to shield for 12 weeks from the date I received my letter.

I have updated work (unable to wfh) who have started the 'clock' started on 15th not 23rd. I've pointed out several times guidance states it starts from the date the letter arrived but they are adamant I am wrong.
Yes it's a few days, but I feel like the they are trying to undermine me.

Any advice?

Can you text your boss a picture with the wording of the letter?

Send them a picture of the letter as PP said.

To be honest, if you're in the shielding group I suspect you'll be given further letters and ultimately will have to continue isolating until there is eventually a vaccine, so this may all become a moot point.