From Aegisub Manual
The shift times tool is a batch processor for timestamps; it adjusts the start/end timestamps of many subtitle lines at once in various ways. It is located under Timing -> Shift Times.
It looks like this:
The left part of the window contain the options.
 Shift by
These controls decide in which direction and by how much each timestamp will be modified.
- Time - How much you want to adjust each timestamp, in hours:minutes:seconds.centiseconds.
- Frames - If you have video loaded, you can specify the adjustment time as a number of frames instead.
- Forward or Backward - controls in which direction the timestamps are adjusted.
These controls decide what lines will be processed.
- All rows - Applies the time shifting to all lines in the script.
- Selected rows - Applies the time shifting only to the selected lines.
- Selection onward - Applies the time shifting to the selected line(s) and all lines below (in the grid) the last selected line.
These controls decide what timestamps will be processed.
- Start and End times - Both start and end times of the affected lines will be modified by the given amount.
- Start times only - Only the start times of the affected lines will be modified. Note that this makes the lines longer (if you shift backwards) or shorter (if you shift forwards) and can even make them have a duration of zero.
- End times only - Only the end times of the affected lines will be modified. Note that this makes the lines longer (if you shift forwards) or shorter (if you shift backwards) and can even make them have a duration of zero.
Note that if a line is shifted so that its start or end time stamp would be negative, that timestamp is zeroed instead. This can be used to clear all timings from an entire script, by shifting backwards by longer than the latest timestamp in the script.
This is a history of all time shiftings you have done since last time you cleared the shift history (with the clear button). The format is a number of fields separated by commas. The fields are:
- Filename of the script (e.g. "example.ass")
- Shift amount and direction (e.g. "0:00:05.00 forward")
- What times were affected, "s" for start, "e" for end, "s+e" for both
- What rows were affected; "sel start-end" for selections, "all" for all rows (e.g. "sel 1-40")