Here's an image of the new date letter for use by the British assay offices as from the 1st January 2018:
Lower case 't' contained within a square punch with clipped corners.
The date letter is now, sadly, voluntary. The London Assay Office, however, still maintain that the date letter is a very important component of the UK hallmark and are the only British assay office that still applies the date letter as standard at no extra charge to the submitter.
The inclusion of the date letter in the hallmark started in 1478 (the same year the Assay Office was founded).
All date punches are destroyed at the end of the year.