Changing Times in MariaDB

Modifying Dates and Times Guide

This guide explores MariaDB functions for performing calculations and modifications on date and time values. Learn to use functions like DATE_ADD, DATE_SUB, TIME_TO_SEC, and SEC_TO_TIME to accurately add or subtract intervals and manage date/time changes that cross midnight or month/year boundaries.

(For foundational knowledge on date and time data types and basic retrieval, please refer to the "Date and Time Handling Guide".)

Calculating Time Across Midnight

When adding hours to a TIME value, calculations might exceed 24 hours. For example, if a task is entered at 23:00 and is promised 2 hours later, a simple addition can be problematic.

Consider an INSERT statement for a tickets table with entered and promised TIME columns:

-- Example: Calculating a promised time 2 hours (7200 seconds) from current time
INSERT INTO tickets (client_id, urgency, trouble, ticket_date, entered, promised)
VALUES ('some_client', 'ASAP', 'Issue details',
        CURDATE(), CURTIME(),
        SEC_TO_TIME(TIME_TO_SEC(CURTIME()) + 7200));

• TIME_TO_SEC(time) converts a time value to seconds.

• SEC_TO_TIME(seconds) converts seconds back to a time format (HHH:MM:SS).

If CURTIME() is 23:00:00 (82,800 seconds), 82800 + 7200 = 90000 seconds. SEC_TO_TIME(90000) would result in 25:00:00. While MariaDB can store this, it doesn't represent a standard clock time for the next day.

Modulo Arithmetic for Time Rollover:

To handle time wrapping around the 24-hour clock (86,400 seconds in a day) for TIME columns, use the modulo operator (%):

-- Corrected calculation for 'promised' TIME, wraps around 24 hours
SEC_TO_TIME((TIME_TO_SEC(CURTIME()) + 7200) % 86400)

If current time is 23:00, (82800 + 7200) % 86400 becomes 90000 % 86400, which is 3600 seconds. SEC_TO_TIME(3600) correctly results in 01:00:00.

Tracking Date Changes with Time: Using DATETIME

The modulo arithmetic above gives the correct time of day but doesn't indicate if the promised time falls on the next calendar day. For calculations where the date might change, it's essential to use DATETIME (or TIMESTAMP) data types.

If your table initially used separate DATE and TIME columns (e.g., ticket_date, entered_time, promised_time), you would typically alter the table to use DATETIME columns (e.g., entered_datetime, promised_datetime) to store both date and time information accurately. This often involves:

1. Adding new DATETIME columns.

2. Populating them by combining the old date and time columns (e.g., using CONCAT(ticket_date, ' ', entered_time)).

3. Dropping the old separate date and time columns. (Always back up your data before such structural changes.)

With DATETIME columns, NOW() can be used to get the current date and time.

Adding Durations with DATE_ADD

The DATE_ADD(date, INTERVAL expr unit) function is the most robust way to add a duration to a date, time, or datetime value. It correctly handles rollovers across days, months, and years.

• date: A DATE, DATETIME, or TIME value.

• expr: The value of the interval to add.

• unit: The unit of the interval (e.g., HOUR, MINUTE, DAY, MONTH, YEAR, etc.).

Adding Hours (handles date change):

If entered and promised are DATETIME columns:

INSERT INTO tickets (client_id, urgency, trouble, entered, promised)
VALUES ('some_client', 'ASAP', 'Issue details',
        NOW(),
        DATE_ADD(NOW(), INTERVAL 2 HOUR));

If NOW() is 2025-06-03 23:00:00, promised will correctly be 2025-06-04 01:00:00.

Adding Combined Hours and Minutes:

Use HOUR_MINUTE as the unit. The expr is a string 'hours:minutes'.

-- Add 2 hours and 30 minutes
DATE_ADD(NOW(), INTERVAL '2:30' HOUR_MINUTE)

If NOW() is 2025-06-03 23:00:00, this results in 2025-06-04 01:30:00.

Date Calculations Across Months and Years with DATE_ADD

DATE_ADD also correctly handles date changes across month and year boundaries, including leap years.

Adding Days:

-- Add 5 days
DATE_ADD(NOW(), INTERVAL 5 DAY)

If NOW() is 2025-02-27, this would result in 2025-03-04 (assuming 2025 is not a leap year).

Adding Combined Days and Hours:

Use DAY_HOUR as the unit. The expr is a string 'days hours'.

-- Add 2 days and 6 hours
DATE_ADD(NOW(), INTERVAL '2 6' DAY_HOUR)

Adding Combined Years and Months:

Use YEAR_MONTH as the unit. The expr is a string 'years-months'.

-- Add 1 year and 2 months
DATE_ADD(NOW(), INTERVAL '1-2' YEAR_MONTH) -- Note: Original text used '1 2', '1-2' is common for YEAR_MONTH

If NOW() is 2025-09-15 23:00:00, this results in 2026-11-15 23:00:00. This type of interval typically does not affect the day or time components directly, only the year and month.

Subtracting Durations

Using DATE_ADD with a Negative Interval:

You can subtract durations by providing a negative value for expr.

-- Subtract 5 days
DATE_ADD(NOW(), INTERVAL -5 DAY)

Using DATE_SUB(date, INTERVAL expr unit):

This function is specifically for subtracting durations.

-- Subtract 5 days
DATE_SUB(NOW(), INTERVAL 5 DAY)

Note: With DATE_SUB, expr is positive for subtraction. A negative expr would result in addition.