Pure Python conversion — no FFmpeg or external tools needed.

Supported inputs: SubRip .srt and WebVTT .vtt files. BOM-prefixed UTF-8 files are handled. Other formats (ASS/SSA, SUB, etc.) are not supported here — convert to SRT first using a tool like Aegisub.

Time shift is in seconds and can be negative. Use to fix subtitles that are consistently early (positive shift) or late (negative shift) compared to the audio. For non-uniform drift (subtitles speeding up over time), this tool can't help — you need a re-timing tool.