Fitting Date, Not Purchase Date
A UV bulb ages only while it is running, so the date that matters is the day it goes into the fitting — not the day it was bought, which might be months earlier if you keep spares. Write the fitting date on the bulb itself with a permanent marker, on the end cap where the heat will not reach it, and log the same date in your records. When you eventually pull a bulb out and wonder how long it served, the answer is printed on it even if every other record has been lost.
What to Log for Each Bulb
Keep one line per bulb per enclosure: the fitting date, the exact lamp type and output rating, which enclosure it serves, and its position if the vivarium runs more than one light source. Add the date it comes out and why — reached its rated life, failed outright, or measured weak — and over a few cycles you build a true picture of how long each type of lamp actually lasts in your setups, which no packaging claim can give you. That history is what turns guesswork about the next purchase into knowledge.
Measure the Fade Instead of Guessing It
A lamp can glow brightly long after its UV output has faded below usefulness, so the calendar alone is a blunt instrument. If you keep species with serious UV needs, a handheld UV meter earns its place: take a reading at the basking spot when a bulb is new, write it in the log beside the fitting date, and re-measure every month or two. The bulb's own numbers then decide its retirement — a lamp holding its output can serve out its life, and one fading early is caught while the animal has lost nothing.
Stagger a Collection's Changes
If you run several enclosures, resist fitting every bulb on the same weekend: stagger the fitting dates and no month ever carries a full set of replacements at once, in cost or in workload. The log makes the staggering visible — a glance down the fitting dates shows which enclosure is next due and which has months in hand. Keep one spare of each lamp type you use, bought when the log says a change is approaching rather than when a bulb has already died.
Dates That Stay With the Animal
Bulb history belongs with the animal's other records, because a vet investigating a bone or appetite problem will want to know how long the current lamp has run. KinsLe's Pet Passport keeps your reptile's records together in one portable packet, so fitting dates, lamp types and meter readings sit alongside the vet history instead of on a sticky note stuck to the vivarium glass.