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Let me see if I can help and please rate 5 stars. The rules are incredibly strict and 14 days means 14 days. The rule states as follows:
Rule 19(2) states that:
(2) If the [appellant] is in the United Kingdom, the notice of appeal must be received not later than 14 days after they are sent the notice of the decision against which the appeal is brought.
The notice must be received within the time limit, not merely sent. However, notice of appeal is lodged in time if it is provided to the Tribunal before midnight on the last day of the 14 day time limit (rule 11(1)), and if the 14th day is not a working day, then it is provided in time if it is provided on the next working day (rule 11(2)). A 'working day' means any day other than a Saturday, Sunday, bank holiday or the period 25th -31st December (rule 1(4)).
I hope this helps and please rate 5 stars.