The whip-round for the person leaving the office for pastures new is traditional, if illogical; you have to buy a present for the person who is deserting you (just as their waste treatment plant in Nuneaton needs snagging) because they have landed a new job, paying more, by saying that their current office does rubbish work.
There is a more recent tradition - unpleasantly American and equally illogical - that every office worker ought to buy a general round of cakes or sweets on their birthday: 'I'll have the French fancies please, the extra large box.' Oh, how festive, what a spree! If you pretend you are diabetic and give enough notice, savouries might be acceptable; at least then someone else will be disappointed too.
The practice is eagerly adopted by the gullible but unimaginative, the greedy but unambitious, those who enthusiastically embrace the long-service carriage clock convention and annually observe Secretaries Day, the drudges who puritanically ignore such well-observed festivals as National Phone In Sick Day.
Going to the pub, with half of your colleagues for the first round and the resident alcoholic for the remainder, used to be thought satisfactorily celebratory.
Directors would know to bail out before you had too many cherry brandies and started complaining that you had not had a proper pay rise since you hit the project manager in a heated debate about raised floors, and that Julian had only been promoted because the MDfancied him.
Just keep it quiet. Celebrate the speeding of the years with your friends, away from work, or, if you can't manage that, alone in a stupor a safe distance from the office.