added timestamp modification to knowledge base

content/knowledge-base/modify-image-timestamp.md

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+date = '2026-03-10T18:37:42+01:00'
+draft = false
+title = 'Modify Image Timestamp'
+type='post'
+tags = ['Linux', 'Photography']
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+Some cameras don't automatically update their internal clock. This leads to images with "wrong" timestamps.
+Modifying the [file metadata](https://en.wikipedia.org/wiki/Metadata) as well as the [Exif metadata](https://en.wikipedia.org/wiki/Exif) can fix this problem.
+
+## Modifying the file metadata
+```bash
+touch -m -d "<date>" "<file_name>"
+```
+- `-m` only modify the modification date
+- `-d "<date>"` use `<date>` instead of the current date 
+- `<file_name>` name of the file to be modified
+
+### Example Script for adding an offset
+The following command adds 20 years and 7 months while subtracting 6 days.
+```bash
+#!/bin/bash
+original_mtime=$(stat -c %y "<file_name>")
+new_mtime=$(date -d "$original_mtime +20 years +7 months -6 days" '+%Y-%m-%d %H:%M:%S')
+touch -m -d "$new_mtime" "<file_name>"
+```
+- `stat -c %y "<file_name>"` reads the current modification timestamp from the file
+- `date -d ...` calculates the new date
+
+## Modifying the Exif metadata
+```bash
+exiftool -P -overwrite_original "-allDates=<date>" "<file_name>"
+```
+- `exiftool` [FOSS](https://en.wikipedia.org/wiki/Free_and_open-source_software) tool for viewing and modifying Exif data ([source](https://exiftool.org/))
+- `-P` to not overwrite the file modification time (file metadata)
+- `allDates=...` modifies the exif metadata dates
+    - can be used with operators (i.e. `+=`, `-=`) for offsets
+
+### Example Script for adding an offset
+The following command adds 20 years and 7 months while subtracting 6 days.
+```bash
+#!/bin/bash
+exiftool -P -overwrite_original "-allDates+=20:7:0 0:0:0" "<file_name>"
+exiftool -P -overwrite_original "-allDates-=0:0:6 0:0:0" "<file_name>"
+```
+
+## Example Script for Combined Modification
+This script modifies all `.JPG` files in a given directory by adding 20 years and 7 months while subtracting 6 days.
+```bash
+#!/bin/bash
+
+find "<dir_name>" -type f -iname "*.JPG" -print0 | while IFS= read -r -d '' file; do
+    echo "Processing $file"
+    
+    #update meta data
+    original_mtime=$(stat -c %y "$file")
+    new_mtime=$(date -d "$original_mtime +20 years +7 months -6 days" '+%Y-%m-%d %H:%M:%S')
+    touch -m -d "$new_mtime" "$file"
+
+    #update exif data
+    exiftool -P -overwrite_original "-allDates+=20:7:0 0:0:0" "$file"
+    exiftool -P -overwrite_original "-allDates-=0:0:6 0:0:0" "$file"
+done
+
+echo "DONE!"
+```
+- `<dir_name>` name of the directory to be modified