# TTML/changeProposal004

From W3C Wiki

# Clarifications for Time Expression Semantics

- The following is a Change Proposal for Issue 199
- Editor: Glenn Adams.
- Date: January 28, 2013.

## Summary

TTML defines three time bases: `media`

, `smpte`

, and `clock`

. The first and last of these are directly related to local real time in accordance to the related media play rate and the related media real start time (i.e., the real time when the related media playback started), parameters not modeled by TTML itself. The relationship between media time (M) and local real time (R) is as follows:

R = playRate * M + realStartTime

where

M ∈ ℜ | 0 ≤ M < ∞ | M in seconds

playRate ∈ ℜ | −∞ < playRate < ∞ | playRate is unit-less

realStartTime ∈ ℜ | 0 ≤ realStartTime < ∞ | realStartTime in seconds, with 0 being start of epoch

For sake of convenience, and without loss of generality, we assume playRate is 1 and realStartTime is 0 for the remainder of this document.