Hi. Glide can already generate the annual VAT return deadline. If you generate a new trigger date field on the client record (link to guidance below) and set the frequency type to annual, then the deadline will be correct. This deadline will only apply to jobs created by this trigger. If you already have an annual trigger, then just change the frequency type from 12 months to 1 year and this will work too.

https://glide.helpscoutdocs.com/article/130-set-up-a-new-trigger-field

Let us know if you have any difficulty.