This closes #2192, made special date format result consistent with Excel

[Copilot]

The timeFromExcelTime function had an off-by-one error when converting Excel dates 1-59 (January 1900 through February 1900) to Go time values. This caused dates to be returned one day earlier than expected.

Problem

When reading Excel date 2.0 (which should represent January 2, 1900), the function incorrectly returned January 1, 1900:

result := timeFromExcelTime(2.0, false)
// Expected: 1900-01-02
// Actual:   1900-01-01 ❌

Root Cause

The issue was in the Julian date conversion logic used for Excel dates ≤ 61. The function uses different conversion methods:

• Julian conversion: for days 0-61 (handles Excel's leap year bug around Feb 29, 1900)
• Gregorian conversion: for days > 61

The Julian conversion was working correctly for:

• Day 0 (Excel epoch: 1899-12-30) ✅
• Days 60-61 (around Excel's leap year bug) ✅

But days 1-59 had an incorrect offset, causing them to be off by one day.

Solution

Applied a targeted fix that adds +1.0 to the Julian date offset specifically for Excel days 1-59:

// Fix for off-by-one error in days 1-59
offset := OFFSET1900
if wholeDaysPart >= 1 && wholeDaysPart <= 59 {
    offset = OFFSET1900 + 1.0
}
date = julianDateToGregorianTime(MJD0, excelTime+offset)

This preserves the correct behavior for days 0, 60, and 61 (which handle Excel's leap year bug correctly) while fixing the offset error for the affected range.

Verification

• timeFromExcelTime(2.0, false) now correctly returns 1900-01-02 ✅
• All existing tests continue to pass ✅
• Added comprehensive test coverage (TestTimeFromExcelTime_Issue2192) for edge cases

Fixes #2192.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.24%. Comparing base (11dcc8f) to head (fbd2325).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2193   +/-   ##
=======================================
  Coverage   99.24%   99.24%           
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  Files          32       32           
  Lines       30528    30562   +34     
=======================================
+ Hits        30297    30331   +34     
  Misses        153      153           
  Partials       78       78           

Flag	Coverage Δ
unittests	99.24% <100.00%> (+<0.01%)	⬆️

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[xuri]

The changes in the date.go will cause incorrect cell value if weekdays, elapsed date, and times in number format code.

I made some changes based on this branch.