2nd/10_Wiki/Topic_Programming/Topic_C/C_Date_Time.md

id, title, category, status, verification_status, canonical_id, aliases, duplicate_of, source_trust_level, confidence_score, created_at, updated_at, review_reason, merge_history, tags, raw_sources, applied_in, github_commit

id	title	category	status	verification_status	canonical_id	aliases	duplicate_of	source_trust_level	confidence_score	created_at	updated_at	review_reason	merge_history	tags	raw_sources	applied_in	github_commit
c-date-time	C Date and Time	Programming_Language	draft	conceptual		time.h struct tm strftime C 날짜와 시간		B	0.86	2026-07-04	2026-07-04			c programming-language w3schools date-time time.h	https://www.w3schools.com/c/c_date_time.php

C Date and Time

🎯 한 줄 통찰 (One-line insight)

struct tm's year and month fields are BOTH offset in ways that will silently produce wrong output if you forget to correct them — tm_year counts years SINCE 1900 (so you must add 1900 to get the real year) and tm_mon is zero-indexed (0 = January, so you must add 1 for a human-readable month) — two unrelated offset conventions bundled into the same struct that both require manual correction before display. [S1]

🧠 핵심 개념 (Core concepts)

• time(&var) / time(NULL) — retrieves the current time as a time_t value. [S1]
• ctime(&time_t_value) — converts a raw time_t into a human-readable string (e.g. "Mon Jun 24 10:15:00 2025"). [S1]
• localtime(&time_t_value) — converts a time_t into a struct tm POINTER, breaking the time into separate fields (year/month/day/hour/minute/second); accessed via -> since it's a pointer, per the earlier Structs & Pointers rule. [S1]
• tm_year offset — stores years SINCE 1900; must add 1900 for the actual calendar year. [S1]
• tm_mon offset — zero-indexed (0=January, 11=December); must add 1 for the conventional 1-12 month number. [S1]
• strftime(buffer, size, format, tm_ptr) — formats a struct tm into a custom string layout (e.g. "%d-%m-%Y %H:%M:%S"), writing into a caller-provided buffer. [S1]

📖 세부 내용 (Details)

• Getting and printing the current time as a readable string: time_t currentTime; time(&currentTime); printf("Current time: %s", ctime(&currentTime));. [S1]
• Breaking time into fields with the required offset corrections: time_t now = time(NULL); struct tm *t = localtime(&now); printf("Year: %d\n", t->tm_year + 1900); printf("Month: %d\n", t->tm_mon + 1); printf("Day: %d\n", t->tm_mday);. [S1]
• Custom-formatted timestamp: char buffer[100]; strftime(buffer, sizeof(buffer), "%d-%m-%Y %H:%M:%S", t); printf("Formatted time: %s\n", buffer);. [S1]

⚖️ 모순 및 업데이트 (Contradictions & updates)

• tm_year와 tm_mon의 오프셋 함정: tm_year는 1900년 기준 경과 연수이므로 실제 연도를 얻으려면 1900을 더해야 하고, tm_mon은 0부터 시작(0=1월)하므로 실제 월을 얻으려면 1을 더해야 한다는 점이 각각 별개의 주의사항으로 명시됨. [S1]

🛠️ 적용 사례 (Applied in summary)

현재 발견된 실제 적용 사례가 없습니다 — 로그 기록, 파일/메시지에 타임스탬프 추가, 실행 시간 측정, 난수 시드 설정 등이 날짜/시간 라이브러리의 대표 활용 목적으로 원문에 직접 나열됨. [S1]

💻 코드 패턴 (Code patterns)

Breaking the current time into fields with the mandatory offset corrections (C):

#include <time.h>
time_t now = time(NULL);
struct tm *t = localtime(&now);
printf("Year: %d\n", t->tm_year + 1900); // tm_year counts from 1900
printf("Month: %d\n", t->tm_mon + 1);    // tm_mon is 0-indexed

✅ 검증 상태 및 신뢰도

• 상태: draft
• 검증 단계: conceptual
• 출처 신뢰도: B (W3Schools — widely used educational reference, not a primary standards body)
• 신뢰 점수: 0.86
• 중복 검사 결과: 신규 생성 (New discovery)

🔗 지식 그래프 (Knowledge Graph)

• 상위/루트: C Tutorial
• 관련 개념: C User Input, C Structs Pointers, C Random Numbers, C Errors
• 참조 맥락: 사용자 입력 및 날짜 섹션 마지막 — 에러 및 디버깅(Errors & Debugging) 섹션으로 이어짐.

📚 출처 (Sources)

• [S1] W3Schools — C Date and Time — https://www.w3schools.com/c/c_date_time.php

📝 변경 이력 (Change history)

• 2026-07-04: Initial draft synthesized from the W3Schools "C Date and Time" page (Astra wiki-curation, P-Reinforce v3.1 format).