Updated documentation on message format for file node-type

documentation/clients/File.md

@@ -77,15 +77,25 @@ If this setting has a non-zero value, the default behaviour is overwritten with
 
 ## File Format
 
-This node-type uses a simple human-readable format to save samples:
+This node-type uses a simple human-readable format to save samples to disk:
 The format is similiar to a conventional CSV (comma seperated values) file.
 Every line in a file correspondents to a message / sample of simulation data.
 The columns of a line are seperated by whitespaces (tabs or spaces).
 The columns are defined as follows:
 
- 1. Seconds in floating point format since 1970-01-01 00:00:00 (aka Unix epoch timestamp: `date +%s`).
- 2. Sequence number
- 3. Up to `MSG_VALUES` floating point values per sample. The values are seperated by whitespaces as well.
+    seconds.nanoseconds+offset(sequenceno)	value0 value1 ... valueN
+
+ 1. The first column contains a timestamp which is composed of up to 4 parts:
+     - Number of seconds after 1970-01-01 00:00:00 UTC
+     - A dot: '.'
+     - Number of nano seconds of the current second (optional)
+     - An offset between the point in time when a message was sent and received (optional)
+     - The sequence number of the message (optional)
+     
+     A valid timestamp can be generated by the following Unix command: `date +%s.%N`.
+     *Important:* The second field is not the fractional part of the second!!!
+
+ 2. Maximum `MSG_VALUES` floating point values per sample. The values are seperated by whitespaces as well.
 
 ### Example