Before WHEN?!?!?!?

I/we knew there was a home-brew can lurking in a kitchen cupboard when we bought the brewing gear at Lakeland©. What we didn't realise, was how long it had been lurking.
Anyway, I've never belonged to the "It goes off on the stroke of midnight on that date." brigade ... Nothing ventured, worse case scenario? I can't abide the flavour, or the yeast's gorn orff.
BTW, have you ever heard of "Sodium percarbonate"?
Neither had I, in fact, shown the formula, I'd probably have said
"How the hell can that exist?"

"Sodium percarbonate is a chemical, an adduct of sodium carbonate and hydrogen peroxide, with formula 2Na2CO3 · 3H2O2.
It is a colorless, crystalline, hygroscopic and water-soluble solid.
Formula: Na2CO3·1.5H2O2"
It's also supplied with the brewing kit, presumably in lieu of the Sodium metabisulphite which I know can exist.

WHAT?!?!?
HOW the great grey green greasy Limpopo can you have 1.5H2O2???????????
Surely its formula must be the original one they quoted. Chemi must have altered VASTLY "sen Ah waz a lad."

Also, probably, as good an example of a screeching need for focus-stacking as I ever produced.