All calculations on utilduck run as JavaScript inlined in the page and execute entirely in your browser — there is no server-side calculation.
Each tool page shows the formula and reference it uses directly in the page content. Tools that rely on an official reference table link to that agency's primary source at the bottom of the page.
Blackjack EV engine. The blackjack strategy calculator computes expected value with exact calculation, not simulation. It uses an infinite-deck model with dealer stands on soft 17 (S17), double after split (DAS) allowed, no resplitting, late surrender, a peek rule, and split aces receive one card each.
For each hand it returns the expected value (EV), the probabilities of a win, push, and loss, and the optimal action — stand, hit, double, split, or surrender.
We checked the engine's optimal actions against published S17/DAS/no-resplit/late-surrender basic strategy charts across all 340 hand-versus-dealer-upcard cells. 339 of 340 matched exactly; the one mismatch has an EV difference of 0.0009, an effective tie between the two actions.
These results assume this specific rule set. A casino table with different rules will produce different correct plays and different EVs. Results do not guarantee any gambling outcome.
Lunar calendar engine. The lunar calendar converter covers the years 1900 to 2100 and converts using the Vietnamese lunar calendar (âm lịch), calculated at UTC+7. Rather than a fixed offset table, it calculates the actual new moon and solar terms astronomically, implementing the algorithm Hồ Ngọc Đức published for the official Vietnamese calendar.
The Vietnamese lunar calendar does not always match the Korean or Chinese lunar calendar. 2007 is a documented boundary case where the Vietnamese calendar (UTC+7) and the Chinese calendar (UTC+8) fall a day apart. Dates outside the 1900–2100 range are not supported.
We verified the engine against the actual dates of 31 Vietnamese Tết (Lunar New Year) celebrations from 1996 to 2026 — including the 2007 boundary case — and all matched. We also ran 100 round-trip solar→lunar→solar conversions across the full 1900–2100 range, all of which succeeded.
Testing and source code. Both engines are published under the MIT license at github.com/Kyunghwan-Sohn/utilduck-engines. Their automated test suites currently report: node blackjack-ev/test.mjs — 24 passed, 0 failed; python3 blackjack-ev/reference.py; node lunar-calendar/test.mjs — 36 passed, 0 failed. The site as a whole is checked with Playwright-based regression tests before each deploy.
What results do not cover. Results are estimates for reference only. They are not medical, legal, tax, or financial advice. Laws and reference tables can change — check the source link on each tool page for the current version.
See Sources and verification for how we choose and verify the reference data behind these calculations.