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

# [[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):
```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).