One way would be to use a regular expression, for example, the following line would retrieve the date string following the text after the 'get'.
var textFileLine = "get rf_da /n Device Timestamp, 6/16/2015 2:14:48 PM";
var lineAfterGetRegEx = /(get)([\w|\s|\n]+)(Device Timestamp,\s)([^"]+)/g;
var lineAfterGetMatch = lineAfterGetRegEx.exec(textFileLine); alert(lineAfterGetMatch[4]);
From there, you can refine the regular expression above to retrieve the date. Here is a handy online regex tester you can use:
https://regex101.com/
For more details, see the following links:
How do you access the matched groups in a JavaScript regular expression?