Does MATLAB datetime arithmetic ignore inserted leap seconds?

Answer

MATLAB’s datetime objects always treat a day as exactly 86 400 SI seconds. Subtracting two datetime values that span a historical leap second returns the calendar‑day difference, not the true elapsed time that includes the extra second. No MATLAB release, nor any supported flag or function, changes this behavior.

Confirmed facts

• The datetime class was introduced in R2014b and, since then, the documentation has explicitly stated that leap seconds are not represented in the internal time scale.
• All arithmetic involving datetime, duration, and calendarDuration uses a uniform 86 400‑second day and ignores any inserted leap seconds.
• Searches of release notes for keywords such as "leap second" reveal no addition of a leap‑second table or a switch to leap‑second aware arithmetic.
• The TimeZone property can be set to "UTC", but this merely selects the uniform UTC time scale; it does not enable leap‑second handling.

Practical verification

% MATLAB example (R2023b or later)
> t1 = datetime(2016,12,31,23,59,59,'TimeZone','UTC');
> t2 = datetime(2017,1,1,0,0,0,'TimeZone','UTC');
> diff = t2 - t1

diff =
 1 days 00:00:01

Although the leap second 2016‑12‑31 23:59:60 existed, the difference is reported as exactly 1 day (86 400 s). The internal representation does not distinguish the extra second.

Implications for applications

If your workflow requires legal or scientific accuracy that must account for leap seconds, you must apply a correction after MATLAB arithmetic. Typical approaches include:

• Maintain a local copy of IERS bulletin B (or equivalent) that lists cumulative leap‑second offsets.
• After computing the raw difference with datetime, adjust the result by adding or subtracting the number of leap seconds that occurred in the interval.
• Alternatively, use a third‑party library that implements a leap‑second aware time scale and convert to/from MATLAB datetime for further processing.

What to check next

Since the behavior is uniform across all supported releases, there is no release‑specific setting to enable. If you are working with a particular MATLAB version, confirm that it is 2014b or newer (the datetime class did not exist before). Otherwise, you can safely assume the leap‑second‑ignorant behavior applies.

Summary table

Release	Leap‑second awareness	Supported flag / function
R2014b – R2026	Ignored (uniform 86 400‑s day)	None