Update 1450-date-timestamp.markdown

Author: Joe7M

_build/reference/1450-date-timestamp.markdown

@@ -1,7 +1,19 @@
 # TIMESTAMP
 
-> TIMESTAMP filename
+> s = TIMESTAMP (filename)
 
-Returns the file last modified date and time.
+Returns the file `filename` last modified date and time as a string. The returned string `s` has the format "YYYY-MM-DD hh:mm AM|PM".
+
+### Example
+
+```
+' Create a file
+open "timetest.txt" for output as #1
+    print #1, 1
+close #1
+
+' Get time stamp
+print timestamp("timetest.txt")     ' Output: 2023-09-03 11:36 AM
+```