Function deltaT

deltaT(year: number, month: number): number
Calculate ΔT (TT - UT) in seconds for a given date

This function uses polynomial approximations that vary by era. The coefficients are empirically derived from historical observations and extrapolated for future dates.

Accuracy:
- Before 1600: ~100-1000 seconds uncertainty
- 1600-1800: ~10-100 seconds
- 1800-1900: ~1-10 seconds
- 1900-2000: ~0.1-1 second (measured values)
- 2000-2050: ~1-10 seconds (predicted)
- After 2050: Increasingly uncertain (extrapolated)
Parameters
- year: number
  Full year (e.g., 2025, -100 for 101 BCE)
- month: number
  Month (1-12, used for interpolation)
Returns number
ΔT in seconds
Example
```
// Modern era (well-measured)
const dt2020 = deltaT(2020, 1); // ~69 seconds

// Historical (estimated)
const dt1000 = deltaT(1000, 1); // ~1600 seconds

// Ancient (extrapolated)
const dt100bce = deltaT(-100, 1); // ~17000 seconds
```
- Defined in time/delta-t.ts:69