Most calendar APIs return a date. This one returns the reasoning.
Below is a capability-by-capability account of the difference, and the evidence behind each row. We compare against the typical behaviour of Panchang APIs generally, not against any named competitor.
Capabilities
What is actually different
Explainability
TathaAstu
Rule, version, evaluator, overlap in minutes and authority on every festival
Typical Panchang API
A date, with no derivation
Why it matters. When a date is disputed — and these dates are disputed — you need to show reasoning, not assert a value.
Kala vyapini evaluation
TathaAstu
Overlap between a tithi interval and a required time-of-day window
Typical Panchang API
Whichever tithi was running at sunrise
Why it matters. Sunrise matching puts Ganesh Chaturthi, Vijayadashami and Maha Shivaratri a day late. We shipped those corrections.
Confidence scoring
TathaAstu
Deterministic score with a label, on every plan including Free
Typical Panchang API
None
Why it matters. A date from a full overlap and a date from a kshaya fallback should not look equally certain.
Rule versioning
TathaAstu
DRAFT to ARCHIVED lifecycle; rules never edited in place
Typical Panchang API
Rules edited or hardcoded
Why it matters. Every response we have ever served remains reproducible from its recorded rule id and version.
Independent verification
TathaAstu
Three tiers on every build: reference, determinism, audit replay
Typical Panchang API
No published verification
Why it matters. Each tier catches a failure the others cannot. Determinism in particular catches a rule change that moved an unrelated date.
Observatory mode
TathaAstu
Raw astronomical intervals attached on request, at no extra cost
Typical Panchang API
Opaque computation
Why it matters. A derivation you cannot check is only marginally better than no derivation.
Published limitations
TathaAstu
Our internal limitations document, verbatim, on a public page
Typical Panchang API
Marketing claims only
Why it matters. You will find the limitations eventually. Better from us than in production.
Interval boundaries
TathaAstu
Start and end instants for tithi, nakshatra, yoga and karana
Typical Panchang API
A label for the day
Why it matters. Without boundaries you cannot schedule anything that has to respect a tithi change mid-morning.
Evidence
The verification behind the claims
Every capability above is checked on every build. These are the results, not targets.
| Tier | What it checks | Scope | Result |
|---|---|---|---|
| Tier A | Reference comparison | 7 festivals × 6 years, spanning 1950–2100 | 42 / 42 |
| Tier B | Determinism lock | Every-day festival hash across 8 years | Locked |
| Tier C | Audit replay | 22 days from the 2026–27 accuracy audit | 26 / 26 |
What we are not claiming
Try it
Verify the claim yourself
Open the playground, set the date to 8 November 2026, and select the Festivals endpoint. You will get Diwali with confidence: 100, confidence_label: FULL_KALA_COVERAGE, and the rule code and version that produced it.
Then ask any other calendar API the same question and see whether it can tell you why.