Offset of MPEG-TS timestamps in seconds. This only affects streams with [WebVttConfiguration](#/Encoding/PostEncodingConfigurationsSubtitlesWebVtt). If set, the X-TIMESTAMP-MAP will be added as described in the [HLS specification](https://datatracker.ietf.org/doc/html/rfc8216#section-3.5). For example, if set to 10 seconds, *X-TIMESTAMP-MAP=MPEGTS:900000,LOCAL:00:00:00.000* will be added after each *WEBVTT* header. The default for TextMuxing is that the X-TIMESTAMP-MAP will not be written. Important to note is that the default for `startOffset` for [TsMuxings](#/Encoding/PostEncodingEncodingsMuxingsTsByEncodingId) and [ProgressiveTsMuxings](#/Encoding/PostEncodingEncodingsMuxingsProgressiveTsByEncodingId) is 10 seconds. If the output of this muxing is used for HLS together with video/audio streams using TsMuxings and ProgressiveTsMuxings, this value should be set to the same `startOffset`.