Short answer: you need to parse the dates and do computations with the result. Will go back for details. Or search for SimpleDateFormat.

Your problem is unrelated to Arduino...

1. String dateStr1 = "2012/05/18 13:00";
2. String dateStr2 = "2012/06/05 17:00";
3. DateFormat df = new SimpleDateFormat("yyyy/MM/dd HH:mm");
4. long date1, date2;
5. try
6. {
7.   date1 = df.parse(dateStr1).getTime();
8.   date2 = df.parse(dateStr2).getTime();
9. }
10. catch (ParseException e)
11. {
12.   e.printStackTrace();
13.   exit();
14.   return;
15. }
16. long delta = date2 - date1; // Milliseconds
17. long hours = delta / 1000 / 3600;
18. long days = hours / 24;
19. hours = hours - days * 24;
20. println("Time spent: " + days + " days and " + hours + " hours");
21. exit();

let's assume I have hour:min:sec stored in c1[0] and want to convert into secs only

• 00:00:15 would give 15
• 00:01:15  ---> 75 or so
• 01:00:05  ---> 3605 or so
  

this:

1. //
2.     DateFormat df = new SimpleDateFormat("ss");
3.     long date1;
4.     try
5.     {
6.       date1 = df.parse(c1[0]).getTime();
7.     }
8.     catch (ParseException e)
9.     {
10.       e.printStackTrace();
11.       exit();
12.       return;
13.     }

works not.... gives me minus something and abs far too high...

who can help me....

Thank you!!!

getTime() returns the number of milliseconds since the epoch, some past reference. So it always be a big number...
Beside, you don't parse the whole string, so it cannot work.
This gives the result you expect:

1. String[] dates =
2. {
3.   "00:00:15",
4.   "00:01:15",
5.   "01:00:05",
6. };
7.  
8. void setup()
9. {
10.   long ref = getTime("00:00:00");
11.  
12.   for (String date : dates)
13.   {
14.     println(int((getTime(date) - ref)/1000));
15.   }
16.   exit();
17. }
18.  
19. DateFormat df = new SimpleDateFormat("HH:mm:ss");
20. long getTime(String date)
21. {
22.     try
23.     {
24.       return df.parse(date).getTime();
25.     }
26.     catch (ParseException e)
27.     {
28.       e.printStackTrace();
29.       return 0;
30.     }
31. }