What Time Formats Are Valid in SMIL clipBegin and clipEnd?

Synchronized Multimedia Integration Language (SMIL) supports two primary time value categories for the clipBegin and clipEnd attributes: Clock Values and SMPTE Timecodes. These attributes allow authors to extract specific sub-clips from continuous media objects such as audio or video files. Depending on the SMIL profile and player implementation, clock values can be expressed in full clock format, partial clock format, or timecount values with metric units, while SMPTE timecodes allow frame-accurate clipping for professional media.

Clock Values

Clock values describe time offsets relative to the start of the referenced media item. SMIL allows three distinct syntax variations for clock values:

• Full Clock Format (hh:mm:ss.fraction): Specifies two digits for hours, two digits for minutes, and two digits for seconds, optionally followed by a decimal point and fractional seconds. For example, 01:15:30.50 indicates 1 hour, 15 minutes, 30 seconds, and 500 milliseconds.
• Partial Clock Format (mm:ss.fraction): Specifies two digits for minutes and two digits for seconds, with an optional fractional second component. For example, 02:45 represents 2 minutes and 45 seconds, while 00:12.75 represents 12.75 seconds.
• Timecount Values (value[metric]): Specifies an integer or floating-point number followed immediately by an optional time unit indicator. If no metric is explicitly provided, the default unit is seconds (s). The supported metrics are:
• Hours (h): e.g., 1.5h (1 hour and 30 minutes)
• Minutes (min): e.g., 45min (45 minutes)
• Seconds (s): e.g., 90s or 90 (1 minute and 30 seconds)
• Milliseconds (ms): e.g., 1500ms (1.5 seconds)

SMPTE Timecode Values

For frame-accurate positioning within professional broadcast and editing formats, SMIL also supports SMPTE (Society of Motion Picture and Television Engineers) timecodes. SMPTE values require the explicit smpte:, smpte-25:, smpte-30:, or smpte-30-drop: prefix to define the exact frame rate and drop-frame specifications:

• Standard SMPTE (smpte:hh:mm:ss:ff): Follows standard 24, 25, or 30 frames-per-second timecodes using colons between all fields, such as smpte:00:01:20:12.
• Drop-Frame SMPTE (smpte-30-drop:hh:mm:ss;ff): Used for NTSC color television broadcasts (29.97 fps) where dropped frames are accounted for, typically delimited with a semicolon before the frame count (e.g., smpte-30-drop:00:10:00;02).

Syntax Rules and Special Values

When applying clipBegin and clipEnd attributes within SMIL elements like <audio>, <video>, or <ref>, specific syntax constraints apply:

• Unsigned Values: The values supplied to clipBegin and clipEnd must always be non-negative.
• No Whitespace: No whitespace is permitted between the numerical digits and their corresponding time metric (for example, 10 s is invalid, whereas 10s is valid).
• Special Identifiers: In certain SMIL profiles, clipBegin can be set to 0 or 0s to indicate the absolute beginning of the media, and clipEnd can accept implementation-specific keywords or be omitted to span until the native media file duration concludes.