Gregorian-Lunar (Lunisolar) Calendars Converter

Converting from Gregorian calendar to Vietnamese Lunar calendar (add -tz 8 for Chinese Lunar calendar), and from Lunar calendar to Gregorian.

Astronomical algorithms from Ho Ngoc Duc's work.

I re-wrote the algorithms in C to implement an easy-to-use CLI application. This requires a compiler that supports C99 or later.

Compilation

In my case, I use gcc 8.3.0, but any other compilers that support C99 may work just fine (have not tested yet).

# Change directory to the directory containing this README
# then run this

gcc -Wpedantic -g src/lunar.c -o lunar

Arguments

• -tz: Time zone in hours, e.g. -tz 7 (default) is for UTC+07:00, -tz 8 is for UTC+08:00.
• -y: Year, e.g. -y 2024 is the year 2024.
• -m: Month from 1 to 12, e.g. -m 7 is July (Gregorian calendar) or the 7th month (Lunar calendar).
• -d: Day of month, from 1 to 31, e.g. -d 21 is the 21st day of the month.
• -l: (Lunar calendar only) Whether the month is leap. When -l does not present, the month is set as not leap, otherwise leap.
• -lms: (Lunar calendar only) Show the month's size ("大" and "小").
• -lm: (Only when displaying Lunar calendar) Display the date in Lunar calendar in "modern notation", e.g.
  • 2024.06.16: The 16th day of the 6th month (non-leap) of the Lunar year whose New Year falls in the Gregorian year 2024.
  • 2025.06+.01: The 1st day of the 6th month (leap) of the Lunar year whose New Year falls in the Gregorian year 2025.
• -r: Converting the date specified by -y, -m, -l and -d in Lunar calendar to Gregorian.
• -ly: Print the first days of all months in the Lunar year specified by -y along with their equivalent Gregorian date.

Usage

Convert date from Gregorian calendar to Lunar calendar:

./lunar 
# Output: "甲辰年六月十六日"
# Explain: Today in Lunar calendar.
# Set "-tz 8" for Chinese calendar.

./lunar -lms
# Output: "甲辰年六月（小）十六日"
# Explain: Today in Lunar calendar -- showing month size.
# Set "-tz 8" for Chinese calendar.

./lunar -lm
# Output: "2024.06.16" 
# Explain: Today in Lunar calendar - modern notation.
# Set "-tz 8" for Chinese calendar.

./lunar -y 2025 -m 7 -d 30
# Output: "乙巳年閏六月初六日" 
# Explain: 2025-07-30 in Gregorian calendar to Lunar.
# Set "-tz 8" for Chinese calendar.

./lunar -y 2025 -m 7 -d 30 -lm
# Output: "2025.06+.06"
# Explain: 2025-07-30 in Gregorian calendar to Lunar - modern notation.
# Set "-tz 8" for Chinese calendar.

Convert date from Lunar calendar to Gregorian calendar:

./lunar -r -y 2025 -m 6 -l -d 6
# Output: 2025.07.30
# Explain: Lunar date 2025.06+.06 is Gregorian date 2025.07.30 in time zone UTC+07:00 (Vietnamese calendar).
# Set "-tz 8" for Chinese calendar.

List first days of all months in a Lunar year with their equivalent Gregorian dates:

./lunar -ly -y 2025            
# Output:
# Lunar   Gregorian
# 2025.01.01      2025.01.29
# 2025.02.01      2025.02.28
# 2025.03.01      2025.03.29
# 2025.04.01      2025.04.28
# 2025.05.01      2025.05.27
# 2025.06.01      2025.06.25
# 2025.06+.01     2025.07.25
# 2025.07.01      2025.08.23
# 2025.08.01      2025.09.22
# 2025.09.01      2025.10.21
# 2025.10.01      2025.11.20
# 2025.11.01      2025.12.20
# 2025.12.01      2026.01.19