Run-hours vs calendar intervals, whichever comes due first — then a printable 12-month plan
| Asset | Interval type | Interval | Usage / day | Calendar cap (days) | Last done |
|---|
Usage/day applies to hour and mile intervals (use a rolling average, not a peak day). Calendar cap is the OEM time limit for usage-based services (oil life, corrosion) — leave at 365 if none. For pure calendar intervals (days or months), the interval itself is the clock.
| Asset / service | Usage interval | Calendar interval | What usually binds |
|---|---|---|---|
| Forklift — engine oil & filter | 250 run-hr | ~6 months | Usage (multi-shift fleets) |
| Rotary-screw compressor — oil | 2,000-4,000 run-hr | 12 months | Either, at 8 hr/day they meet |
| Diesel generator — oil & filter | 200-250 run-hr | 12 months | Calendar (backup duty) |
| Fleet vehicle — synthetic oil | 5,000-7,500 mi | 12 months | Miles for commuters |
| Rooftop HVAC — filters | — | 30-90 days | Calendar |
| Fire extinguisher (NFPA 10) | — | Monthly visual · annual service · 6-yr teardown · 12-yr hydro | Calendar, always |
| Overhead crane (OSHA 1910.179) | — | Frequent daily-monthly · periodic annual | Calendar, mandated |
Typical manufacturer-recommended ranges for planning — your OEM manual, duty cycle, and local codes govern. The pattern to internalize: assets that run hard bind on hours or miles; assets that wait bind on the calendar. Schedule rules that ignore one of the two clocks schedule the wrong day about half the time.
Most maintenance calendars fail on one subtlety: an interval isn't one number, it's a race between two. This template runs the race per asset, every time a date changes, and shows which clock won — so the forklift gets serviced on hour-meter math and the generator on calendar math, from one screen.
For usage intervals (hours or miles): days until usage-due = interval ÷ average usage per day; days until calendar-due = the calendar cap; binding days = the smaller of the two. For calendar intervals: months × 30.44 (or days as given). Next-due date = last completed + binding days. Services per year = 365 ÷ binding days. Status: overdue if next-due is past, due-soon within 30 days, otherwise scheduled.
Enter each asset with its OEM interval, a realistic average usage per day (log actual run-hours for a month if you're guessing), and the date the service was last completed. The next-due column, status flags, and month-by-month plan recompute live. When a service is finished, update last-done and the year re-flows. Print the 12-month view for the shop wall; the QR keeps the wall version connected to the live calculator.
Forklift FL-3: 250-hour service, 6 hr/day, 180-day calendar cap, last done August 1, 2026. Usage clock: 250 ÷ 6 = 41.7 days → September 12. Calendar clock: 180 days → January 28, 2027. Usage wins, so the service lands September 12 — about 13 days out as of late August. That's 365 ÷ 41.7 = 8.8 services a year on this asset alone, which is why multi-shift forklift fleets run their own PM calendars.
Now the counter-example sitting in the same defaults: generator GEN-2, 200-hour oil service at 1.5 hr/day average. Usage says 133 days from March 10 — July 21 — while the calendar cap says March 2027. But July 21 has passed: the row flags 40 days overdue. A generator that "barely runs" fooled the calendar-only spreadsheet; the two-clock math catches it. Fleet truck T-14 (7,500 mi at 60 mi/day) lands September 22, and the rooftop unit on its 3-month filter cycle lands October 31 — all five assets together total about 19.9 services a year, which is the number that turns a schedule into a staffing plan.
Most OEM services have two limits — usage (hours or miles) and time (days or months) — and the rule is service at whichever comes due first. Days until the usage limit = interval ÷ average usage per day. A 250-hour forklift service at 6 hours a day is 41.7 days out; if the calendar limit is 6 months, usage still binds. Compare both dates and schedule the earlier one.
Because oil, coolant, belts, and seals degrade with time, not just run-hours. A backup generator logging 150 hours a year takes 16 months to reach a 200-hour oil interval, but the fluid chemistry and moisture contamination don't wait — so OEM schedules say 200-250 hours or one year, whichever comes first.
Per asset: the asset ID, the service, its interval (usage and/or calendar), average usage per day, the date it was last completed, and who owns it. The schedule then needs the computed next-due date for each line, a status flag (overdue / due soon / scheduled), and a month-by-month view so the annual workload levels instead of stacking.
Compare the computed next-due date to today. This template flags anything past due in red and anything within 30 days as due-soon. The 30-day window is the planning cushion: enough lead to order parts and schedule a tech, short enough that nothing hides.
Yes — the Print button produces the full annual schedule grouped by month, with QR codes so the plan on the wall links back to the live calculator. No account, nothing uploaded; the math runs in your browser.