STEM
CalendarForge
Why does Tết move? Why does Ramadan slide earlier every year? Because the sun and the moon keep two clocks that don’t divide evenly — and you can do the arithmetic. Predict how far a solar, a lunar and a lunisolar calendar drift against the seasons, see the arithmetic, then drift any calendar across the centuries in the Calendar lab. Real, tiny, deterministic maths — no internet. Ages 9–14.
▶ Play it nowBuilt for sittings of about 20 minutes on the screen. That is our design budget, not a measurement.
Meet your mentor
Every Spark & Anvil app has a friendly mentor character that demonstrates, asks, and patiently scaffolds learning.
Mentored by Metone
#4a5bd0 Characters who teach
Meet the cast
CalendarForge's crew ARE the four keepers of a calendar — the cast IS the curriculum: each character's habit IS one calendar's compromise with the sky. Sol keeps the sun's year and tunes the leap rule so the seasons never slip; Luna follows the moon faithfully and lets the seasons drift ~11 days a year (the honest cost of a pure-lunar calendar); Drift is the quiet accountant, adding up the fractions of a day nobody notices until, centuries on, the calendar is ten days out. Mentor Metone knows the 19-year secret — slip a whole extra month in seven years out of nineteen and the moon and the sun come back in step — which is exactly why Tết, Ramadan and Easter move the way they do. Nothing leaves the device.
Sol
Keeps the sun's tropical year and tunes the leap rule (4 / 100 / 400) so a day's error against the seasons takes millennia to build
Luna
Follows the moon month by month; a pure-lunar year is ~11 days short of a solar year, so her dates drift earlier and earlier — the honest cost of tracking the moon alone
Metone
Knows the Metonic secret — 19 solar years ≈ 235 moon-months — so seven years in nineteen get a 13th leap MONTH to keep a lunisolar calendar with the seasons
Drift
The quiet accountant who adds up the fractions of a day nobody notices, until centuries later the calendar is ten days out and needs a correction
What's inside
Learning goal
Why does Tết move? Why does Ramadan slide earlier every year? Because the sun and the moon keep two clocks that don’t divide evenly — and you can do the arithmetic. Predict how far a solar, a lunar and a lunisolar calendar drift against the seasons, see the arithmetic, then drift any calendar across the centuries in the Calendar lab. Real, tiny, deterministic maths — no internet. Ages 9–14.
Mentored by Metone — no data leaves the device.
How CalendarForge handles your kid's data
- ✅ All progress, settings, and AI-generated content stays on the device
- ✅ No analytics, no tracking, no third-party SDKs
- ✅ Free — no ads, no in-app purchases
- ✅ No personal information collected from children — in line with COPPA (updated by the 2026 FTC amendments)
- ✅ No chat with strangers — play-together rooms use a short code and preset emotes, never typed messages