Optimizing WHERE condition for TIMESTAMP field in MySQL SELECT statement
For the specific query:

select sum(diff_ms) 
from writetest_table 
where time_on &gt; '2015-07-13 15:11:56' ;     -- use single quotes, not double


an index on (time_on, diff_ms) would be the best option. So, if the query runs often enough or its efficiency is crucial to your application, add this index:

ALTER TABLE writetest_table 
  ADD INDEX time_on__diff_ms__ix      -- pick a name for the index
    (time_on, diff_ms) ;





(Not related to the question)
  And really, change the table's engine to InnoDB. It's 2015 and MyISAM's funeral was some years ago.
(/rant)