What does */15 * * * * mean?

Every 15 minutes. The parser expands the star with a step of 15 into minutes 0, 15, 30 and 45, and leaves the hour, day of month, month and day of week open, so it fires on the quarter hour of every hour of every day. Counting from 05:07 on a Monday in America/New_York, the first four fire times are 05:15, 05:30, 05:45 and 06:00.

How do I write 09:30 every weekday?

30 9 * * 1-5. Minute 30, hour 9, every day of the month, every month, and days of the week 1 through 5. This page reads that back as At 09:30 on Monday through Friday. MON-FRI works too: three-letter names are accepted in the month and day of week fields, in any case, though a full name like MONDAY is refused with a pointer to MON, because standard cron only takes the three-letter form.

Why does 0 9 1 * 1 fire on Mondays as well as on the 1st?

Because of the day-of-month and day-of-week rule in standard cron, which is where most cron surprises come from. Vixie cron flags a day field whose text begins with a star. If either field begins with a star, both have to match. If neither does, the job runs when either one matches. So 0 9 1 * 1 fires on the 1st of the month and on every Monday, while 0 9 */2 * 1 keeps the AND rule and fires only on Mondays that fall on an odd day of the month. The description on this page says OR in those words whenever the rule is in play.

Does this read the six-field cron expression with seconds?

No, and it says so instead of guessing. Six fields is the Quartz and Spring scheduler form, which puts a seconds field in front of the five UNIX fields, and a Quartz expression can also carry L, W, # and ?. Paste one here and the page names the dialect and tells you to drop the seconds field. The @daily and @hourly shortcuts are named as well, with their five-field forms, 0 0 * * * and 0 * * * *. @reboot has no five-field form and no fire times, because it runs when the machine starts.

Which clock and which timezone are the next run times in?

The timezone you pick in the readout, and the clock your browser reported when the page opened. Both are printed under the list, and the Count from now button counts again from the current moment. That is a preview, not a promise about your server: cron runs in the timezone the machine or the crontab is set to, so pick the zone that machine keeps and compare.

What happens to a schedule on the day the clocks change?

The page shows it rather than hiding it. In spring an hour disappears, so a time such as 02:30 does not exist on that date in that zone. The row is marked as a gap, it names the jump, and it lists the moment the clock jumps instead of dropping the run in silence. In autumn an hour repeats, so 01:30 happens twice. That row is marked as a fold and listed once, at the first of the two, which is what a cron daemon does with a fixed time. Daemons differ over the gap, which is why the row says so.

How far ahead does it look, and how long an expression will it read?

The search runs to 3,653 days ahead, about ten years, and stops there. A schedule that fires rarely, such as one pinned to 29 February, returns the dates it found inside that window and says how many rather than padding the list, and an expression that can never come round, such as 30 February, says exactly that. Expressions up to 200 characters are read.

Does anything leave my browser?

No. The parsing, the sentence and the fire times are all worked out in this tab, there is no account and nothing is uploaded. The expression and the timezone are kept in your own browser so they are still here when you come back, and the Start over button above the tool forgets them.

Cron Expression Generator

Type the five fields and this page shows the expression, the sentence it means and the next ten times it fires, all from one parser, so the words underneath always describe the fields on screen. It reads standard five-field UNIX cron, the Vixie dialect that Debian, Ubuntu and cronie ship, and it names anything outside that dialect rather than guessing: six fields is the Quartz and Spring form with seconds in front, and L, W, # and ? are Quartz extensions, so each is refused by name with the five-field equivalent where there is one. The rule most generators leave unsaid is written into the description here: when day-of-month and day-of-week are both restricted, standard cron fires when either matches, so 0 9 1 * 1 reads as At 09:00 on day 1 of the month OR on Monday, whichever comes first. Fire times are wall-clock times in the timezone you choose, counted from the clock your browser reports, which means a time inside a daylight-saving gap is named as a gap and an hour that repeats in autumn fires once.

fields5 fields

minute0, 15, 30, 45

hourevery

day of monthevery

monthevery

day of weekevery

timezonereading

cronreading the clock

*/15 * * * *

Every 15 minutes

fire times appear once this browser reports its clock

Expression

Reads as

Every 15 minutes

Timezone

Next run

reading

*/15 * * * *. Every 15 minutes.

Both day fields are open, so every day matches.

*/15 * * * *five fields
every 15 minutes, on the quarter hour
0 9 1 * 1day-of-month and day-of-week both set
the 1st OR any Monday, not both required
60 * * * *minute 60
refused, minutes run 0 to 59

What each field takes

Field	Range	Takes
minute	0 to 59	a star, a list like 0,30, a range like 0-29, or a step like */15
hour	0 to 23	a star, a list like 9,17, a range like 9-17, or a step like */2
day of month	1 to 31	a star, a list like 1,15, a range like 1-7, or a step like */2
month	1 to 12	numbers, or the three-letter names JAN to DEC
day of week	0 to 7	numbers, or the three-letter names SUN to SAT, and 0 and 7 are both Sunday

Every field takes a star for all values, a number, a list such as 1,15, a range such as 9-17, a step on a star such as the one in the quarter-hour example, and a step on a range such as 9-17/4. The fire times above are the first 10 this expression produces after the clock shown, searching up to 3,653 days ahead and saying so when a schedule is rarer than that. An expression up to 200 characters is read. The expression and the zone stay in this browser so they are here when you come back, and the Start over button above the tool forgets them.

The dialect. Standard five-field UNIX cron, the Vixie dialect Debian, Ubuntu and cronie ship: minute, hour, day of month, month, day of week, with both 0 and 7 meaning Sunday. It does not read the six-field Quartz and Spring form with seconds in front, the Quartz extensions L, W, # and ?, or the @daily and @reboot shortcuts. Each of those is refused by name, with the five-field equivalent where there is one.

Accuracy. The expression is built and read back by the same parser, so the sentence underneath always describes the five fields on screen. When day-of-month and day-of-week are both restricted, standard cron fires on either, and the description says OR in those words. Next runs are shown in the timezone you pick, and a time that falls in a daylight-saving gap is named as one rather than quietly skipped.

Common questions

What does */15 * * * * mean?: Every 15 minutes. The parser expands the star with a step of 15 into minutes 0, 15, 30 and 45, and leaves the hour, day of month, month and day of week open, so it fires on the quarter hour of every hour of every day. Counting from 05:07 on a Monday in America/New_York, the first four fire times are 05:15, 05:30, 05:45 and 06:00.
How do I write 09:30 every weekday?: 30 9 * * 1-5. Minute 30, hour 9, every day of the month, every month, and days of the week 1 through 5. This page reads that back as At 09:30 on Monday through Friday. MON-FRI works too: three-letter names are accepted in the month and day of week fields, in any case, though a full name like MONDAY is refused with a pointer to MON, because standard cron only takes the three-letter form.
Why does 0 9 1 * 1 fire on Mondays as well as on the 1st?: Because of the day-of-month and day-of-week rule in standard cron, which is where most cron surprises come from. Vixie cron flags a day field whose text begins with a star. If either field begins with a star, both have to match. If neither does, the job runs when either one matches. So 0 9 1 * 1 fires on the 1st of the month and on every Monday, while 0 9 */2 * 1 keeps the AND rule and fires only on Mondays that fall on an odd day of the month. The description on this page says OR in those words whenever the rule is in play.
Does this read the six-field cron expression with seconds?: No, and it says so instead of guessing. Six fields is the Quartz and Spring scheduler form, which puts a seconds field in front of the five UNIX fields, and a Quartz expression can also carry L, W, # and ?. Paste one here and the page names the dialect and tells you to drop the seconds field. The @daily and @hourly shortcuts are named as well, with their five-field forms, 0 0 * * * and 0 * * * *. @reboot has no five-field form and no fire times, because it runs when the machine starts.
Which clock and which timezone are the next run times in?: The timezone you pick in the readout, and the clock your browser reported when the page opened. Both are printed under the list, and the Count from now button counts again from the current moment. That is a preview, not a promise about your server: cron runs in the timezone the machine or the crontab is set to, so pick the zone that machine keeps and compare.
What happens to a schedule on the day the clocks change?: The page shows it rather than hiding it. In spring an hour disappears, so a time such as 02:30 does not exist on that date in that zone. The row is marked as a gap, it names the jump, and it lists the moment the clock jumps instead of dropping the run in silence. In autumn an hour repeats, so 01:30 happens twice. That row is marked as a fold and listed once, at the first of the two, which is what a cron daemon does with a fixed time. Daemons differ over the gap, which is why the row says so.
How far ahead does it look, and how long an expression will it read?: The search runs to 3,653 days ahead, about ten years, and stops there. A schedule that fires rarely, such as one pinned to 29 February, returns the dates it found inside that window and says how many rather than padding the list, and an expression that can never come round, such as 30 February, says exactly that. Expressions up to 200 characters are read.
Does anything leave my browser?: No. The parsing, the sentence and the fire times are all worked out in this tab, there is no account and nothing is uploaded. The expression and the timezone are kept in your own browser so they are still here when you come back, and the Start over button above the tool forgets them.

The expression is built and read back by the same parser, so the sentence underneath always describes the five fields on screen. When day-of-month and day-of-week are both restricted, standard cron fires on either, and the description says OR in those words. Next runs are shown in the timezone you pick, and a time that falls in a daylight-saving gap is named as one rather than quietly skipped.