Cylinders require testing on a cyclic basis. I run a set of twins, three stages, a suit bottle and a couple of ropey old 15s as an air bank for filling air guns, occasionally running air tools, filling the suit bottle when I forget, etc.
I have figured out the cost and frequency of each cylinder's service/test requirements and worked it out as a monthly amount, along with the cost of servicing regs, replacing hoses, annual memberships, insurance, etc. That amount goes, every month, from my current account into an online saver account. The amount needed for the diving upkeep fund is reviewed every six months and would only change if I buy more cylinders. Even then, the cost of a hydro plus O2 clean (£56), a visual plus O2 clean (£45) and 2x O2 cleans (£29 each) over 5 years (£159) spread over 60 months is £2.65.
If a cylinder is 6 months old by the time you receive it, that's less than £16. And here's the newsflash... The cost of ownership, over the same 5 year period is EXACTLY THE SAME: One Hydro+, one Vis+, 2 O2 cleans = £159.
Unless you are planning on selling your cylinder on before it needs testing, what does it matter at what point in the cycle you buy it?