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The editor’s authoritative timeline unit is the integer composition frame. Timecode, seconds, milliseconds, pixel positions, media timestamps, and caption-token times are views or mappings of that frame domain.
In the examples below, the draft runs at 30 fps. One frame is 1 / 30 second, or approximately 33.333 ms.

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Frames and time

For zero-based frame f at frame rate fps:
To map a time to a frame, the rounding policy matters:
Editing boundaries must land on integer frames. If an external timestamp falls between boundaries, choose the rounding rule that preserves the intended semantic edge.

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Half-open layer intervals

A layer with:
is active on the half-open interval:
Its first playable frame is s; its last playable frame is s + d - 1; its right edit edge is s + d. This convention lets adjacent clips meet exactly: [120, 210) followed by [210, 270) has neither a gap nor an overlapping frame.

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Timecode display

For non-drop-frame HH:MM:SS:FF timecode:
At 30 fps, frame 3,725 is:
The frame field is a frame count within the displayed second, not a decimal fraction. At 25 fps, valid values are 00…24; at 30 fps, 00…29.

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Composition duration

The minimum duration needed to include every layer is the maximum right edge:
The last playable frame is therefore composition duration - 1. The duration readout describes the edge after that last frame.

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Playhead versus edge

The playhead represents a frame to evaluate. Trim handles represent edges between frames. At the final visible frame, the evaluation frame can be duration - 1 while the right edge is duration. This distinction explains why:
  • End seeks the last playable frame, not the frame after the draft.
  • a right trim can end at edge 210 while the last visible sample is frame 209;
  • seeking to a selected clip’s end uses its last playable frame for preview.

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Split math

Split a layer [s, e) at interior frame boundary p, where s < p < e:
For source-backed media, exact continuity means advancing the right output’s source start by the source time consumed by the left output:
At 1×, this reduces to the elapsed composition time. For non-1× media, frame-step and listen across the boundary before continuing the edit. If exact continuity is critical, perform the structural split at 1×, verify the boundary, and then apply and review the intended speed on each result.

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Trim math

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Trim the start

Move the left edge from s to p:
This preserves the same source sample at every unchanged composition frame.

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Trim the end

Move the right edge from e to p:

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Move without trimming

Move a layer by n frames:

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Playback-rate mapping

For an active media layer at composition frame f:
At 2×, one composition frame advances 2 / fps source seconds. At 0.5×, two composition frames advance 1 / fps source seconds. Draft FPS remains unchanged.
An animated GIF can restart its animation after a left trim or split. Treat every new GIF boundary as visually unverified until you play and frame-step across it.
Changing playback rate can make the requested source range exceed the available media. Inspect the layer’s final frames and audio tail after changing rate or trimming near a source boundary.

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Caption timestamps

Caption tokens commonly carry millisecond start and end timestamps. To determine whether a token is active at frame f, the evaluator maps the composition-local time into milliseconds and compares it with token intervals.
A token starting at 2,500 ms maps to frame 75 at 30 fps. A timestamp such as 2,517 ms falls between frames 75 and 76; the evaluator must consistently apply its boundary rule. For editorial alignment, judge the visual result on the nearest frames rather than expecting arbitrary millisecond precision from a frame-based render. Caption page duration is entered in milliseconds with a minimum of 200 ms and 100 ms steps. The visual page transition still resolves against composition frames.

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Keyframe timing

Keyframes are stored at whole composition frames. For adjacent keyframes at f0 and f1 with values v0 and v1:
The destination keyframe provides the easing for the segment leading into it. There is no subframe keyframe between integer frames.

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Audio fades

Let a layer start at s, end at edge e, fade-in length fi, and fade-out length fo:
The fade handles preserve at least a one-frame separation when they approach one another. This prevents the transitions from crossing and keeps a valid middle interval.

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Decibels and linear gain

For editor volume dB:
Examples: Amplitude is not perceived loudness, and clipping depends on the mixed signal. Monitor the combined result.

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Timeline pixels and snapping

Timeline pixels are a view mapping, not authored time:
The snapping threshold is converted from a small screen-pixel tolerance into frames at the current zoom. Consequently, the number of frames inside the snap tolerance can change as you zoom while the pointer tolerance remains visually practical.

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Worked frame-rate example

Suppose the opening shot in your draft is:
  • It occupies [0, 180), or six seconds.
  • Its last playable frame is 179.
  • Splitting at frame 72 creates durations 72 and 108.
  • The right half begins at source time 3.4 s (1.0 + 72 / 30).
  • Trimming the right half’s start from 72 to 87 advances its source start from 3.4 s to 3.9 s (15 / 30 = 0.5 s).
  • Nudging that trimmed half ten frames later changes only its composition from; its source start remains 3.9 s.

Timeline fundamentals

Apply the frame model to clips, tracks, and edit points.

Keyframe lanes

Apply composition-frame timing to animation tracks, values, and interpolation.