The glut does actually expire.
It just takes over a year or so....
It does store better in cool dark areas.
Light can degrade it I believe.
You can tell it is losing potency by the smell(you're not supposed to smell it but it does smell less as it ages.)
I buy glut by the gallon which last me 1/2-a year to a year, depending on hiow much I use.
It does smell weaker over time to me.
Mine is in an opaque white bottle(metricide).
Here is a link from TPT
Metricide 14 to replace the Excel dosing - Page 6