Salesforce Cron Expression for Schedulling class

CRON JOB and CRON String 

The System.Schedule method takes three arguments: a name for the job, an expression used to represent the time and date the job is scheduled to run, and the name of the class. This expression has the following syntax:

1	Seconds Minutes Hours Day_of_month Month Day_of_week Optional_year

Salesforce schedules the class for execution at the specified time. Actual execution may be delayed based on service availability.

The System.Schedule method uses the user's timezone for the basis of all schedules.

The following are the values for the expression:

Name	Values	Special Characters
Seconds	0–59	None
Minutes	0–59	None
Hours	0–23	None
Day_of_month	1–31	, - * ? / L W
Month	1–12 or the following: JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC	, - * /
Day_of_week	1–7 or the following: SUN MON TUE WED THU FRI SAT	, - * ? / L #
optional_year	null or 1970–2099	, - * /

The special characters are defined as follows:

Chars	Description
,	Delimits values. For example, use JAN, MAR, APR to specify more than one month.
-	Specifies a range. For example, use JAN-MAR to specify more than one month.
*	Specifies all values. For example, if Month is specified as *, the job is scheduled for every month.
?	Specifies no specific value. This is only available for Day_of_month and Day_of_week, and is generally used when specifying a value for one and not the other.
/	Specifies increments. The number before the slash specifies when the intervals will begin, and the number after the slash is the interval amount. For example, if you specify 1/5 for Day_of_month, the Apex class runs every fifth day of the month, starting on the first of the month.
L	Specifies the end of a range (last). This is only available for Day_of_month and Day_of_week. When used with Day of month, L always means the last day of the month, such as January 31, February 29 for leap years, and so on. When used with Day_of_week by itself, it always means 7 or SAT. When used with aDay_of_week value, it means the last of that type of day in the month. For example, if you specify 2L, you are specifying the last Monday of the month. Do not use a range of values with L as the results might be unexpected.
W	Specifies the nearest weekday (Monday-Friday) of the given day. This is only available for Day_of_month. For example, if you specify 20W, and the 20th is a Saturday, the class runs on the 19th. If you specify 1W, and the first is a Saturday, the class does not run in the previous month, but on the third, which is the following Monday. Use the L and W together to specify the last weekday of the month.
#	Specifies the nth day of the month, in the format weekday#day_of_month. This is only available for Day_of_week. The number before the # specifies weekday (SUN-SAT). The number after the # specifies the day of the month. For example, specifying 2#2 means the class runs on the second Monday of every month.

The following are some examples of how to use the expression.

Expression	Description
0 0 13 * * ?	Class runs every day at 1 PM.
0 0 22 ? * 6L	Class runs the last Friday of every month at 10 PM.
0 0 10 ? * MON-FRI	Class runs Monday through Friday at 10 AM.
0 0 20 * * ? 2010	Class runs every day at 8 PM during the year 2010.

In the following example, the class proschedule implements the Schedulable interface. The class is scheduled to run at 8 AM, on the 13th of February.

1	proschedule p = new proschedule();

2	String sch = '0 0 8 13 2 ?';

3	system.schedule('One Time Pro', sch, p);

Using the System.scheduleBatch Method for Batch Jobs

You can call the System.scheduleBatch method to schedule a batch job to run once at a specified time in the future. This method is available only for batch classes and doesn’t require the implementation of the Schedulable interface. This makes it easy to schedule a batch job for one execution. For more details on how to use the System.scheduleBatchmethod, see Using the System.scheduleBatch Method.