Create permanent unique ID when another cell is populated

Hello

I need to automatically populate a column of cells (X4 - X3000) with a unique ID (21/001, 21/002, 21/003 etc) based off when another cell has been populated with a date (W4 - W3000).

The dates in W4 - W3000 will be added out of sequence and I don't want this to affect the unique ID of X4 - X3000.
Hope I've explained this adequately, an example of the desired affect is printed below, any help would be greatly received.