The other answer suggest this syntax indicates increments. Now I've set up a test with this expression 1/12
on the minutes field:
1/12 * * * * date >> ~/crontest.tmp
If it would increment, then it would run on these minutes of every hour: 1, 13, 25, 37 and 49. But the results so far are:
Mon Aug 24 17:01:01 CEST 2015
Mon Aug 24 18:01:01 CEST 2015
Mon Aug 24 19:01:01 CEST 2015
Mon Aug 24 20:01:01 CEST 2015
Mon Aug 24 21:01:01 CEST 2015
Mon Aug 24 22:01:01 CEST 2015
Mon Aug 24 23:01:01 CEST 2015
Tue Aug 25 00:01:01 CEST 2015
Tue Aug 25 01:01:01 CEST 2015
Tue Aug 25 02:01:01 CEST 2015
Tue Aug 25 03:01:01 CEST 2015
Tue Aug 25 04:01:01 CEST 2015
Tue Aug 25 05:01:01 CEST 2015
Tue Aug 25 06:01:01 CEST 2015
Tue Aug 25 07:01:01 CEST 2015
Tue Aug 25 08:01:01 CEST 2015
Tue Aug 25 09:01:01 CEST 2015
If you'd use an expression like */12
then it would run on "every minute that is divisible by 12": 12, 24, 36, 48 (0 or 60 included?)
So my guess would be that 1/12
would be "every 1st minute of every hour that is divisible by 12", which is never, so it will fall back to "every 1st minute of every hour".
And thus 1/12
in the hours field would run every day at 01:00 AM.