Enter the date you installed your UVB bulb (my pick: Zoo Med ReptiSun 10.0) and its type, and this gives you an exact replace by date rather than the usual vague range.
The problem with UVB is that it fails invisibly. The bulb keeps producing visible light long after the UVB output has dropped below anything useful, so there is no point at which the bulb looks dead. Most keepers replace far too late.
UVB output decays long before the bulb stops emitting visible light — replace on schedule, not when it burns out. A solar meter is the only way to know for certain.
How the date is calculated
Each bulb type has a rated useful UVB life in months. The tool adds that to your install date and returns the result, flagging it as overdue if the date has passed and as due soon if it falls within the next 30 days.
Compact and coil bulbs generally have the shortest useful life. Linear tubes last longer, and mercury vapour bulbs longer still. The tool uses the type you select rather than one blanket figure.
Why a bulb that still lights up can be useless
Visible light and UVB come from different parts of the output, and they do not decay together. A bulb at six months can look identical to a new one and be producing a fraction of the UVB.
Without adequate UVB a turtle cannot synthesise vitamin D3, which means dietary calcium goes unused. The result is metabolic bone disease, which is common, slow to appear, and largely irreversible once the shell has deformed.
Common questions
Can I just test the bulb instead?
Only with a UV meter, which is the reliable method but costs more than several replacement bulbs. For most keepers a calendar reminder is the practical answer.
Does a window count as UVB?
No. Standard glass blocks effectively all UVB. A turtle sitting in a sunny window is getting warmth and no usable UV at all.
Does distance matter?
Yes, a great deal. UVB intensity falls off sharply with distance, so a correctly aged bulb mounted too far from the basking spot still underdelivers. Follow the manufacturer’s stated distance for the bulb, and check your basking area setup puts the animal within it.