Cronos is a .NET library for parsing Cron expressions and calculating next occurrences. It was designed with time zones in mind, and intuitively handles Daylight saving time (also known as Summer time) transitions as in *nix Cron.

Please note this library doesn't include any task/job scheduler, it only works with Cron expressions.

• Supports standard Cron format with optional seconds.
• Supports non-standard characters like L, W, # and their combinations.
• Supports reversed ranges, like 23-01 (equivalent to 23,00,01) or DEC-FEB (equivalent to DEC,JAN,FEB).
• Supports time zones, and performs all the date/time conversions for you.
• Does not skip occurrences, when the clock jumps forward to Daylight saving time (known as Summer time).
• Does not skip interval-based occurrences, when the clock jumps backward from Summer time.
• Does not retry non-interval based occurrences, when the clock jumps backward from Summer time.
• Contains 1000+ unit tests to ensure everything is working correctly.

This section explains how Cron expressions should be converted, when moving to Cronos.

Cronos is distributed as a NuGet package, you can install it from the official NuGet Gallery. Please use the following command to install it using the NuGet Package Manager Console window.

PM> Install-Package Cronos

We've tried to do our best to make Cronos API as simple and predictable in corner cases as possible. So you can only use DateTime with DateTimeKind.Utc specified (for example, DateTime.UtcNow), or DateTimeOffset classes to calculate next occurrences. You can not use local DateTime objects (such as DateTime.Now), because this may lead to ambiguity during DST transitions, and an exception will be thrown if you attempt to use them.

To calculate the next occurrence, you need to create an instance of the CronExpression class, and call its GetNextOccurrence method. To learn about Cron format, please refer to the next section.

using Cronos;

CronExpression expression = CronExpression.Parse("* * * * *");

DateTime? nextUtc = expression.GetNextOccurrence(DateTime.UtcNow);

The nextUtc will contain the next occurrence in UTC, after the given time, or null value when it is unreachable (for example, Feb 30). If an invalid Cron expression is given, the CronFormatException exception is thrown.

It is possible to specify a time zone directly, in this case you should pass DateTime with DateTimeKind.Utc flag, or use DateTimeOffset class, that's is smart enough to always point to an exact, non-ambiguous instant.

CronExpression expression = CronExpression.Parse("* * * * *");
TimeZoneInfo easternTimeZone = TimeZoneInfo.FindSystemTimeZoneById("Eastern Standard Time");

DateTime?       next = expression.GetNextOccurrence(DateTime.UtcNow, easternTimeZone);
DateTimeOffset? next = expression.GetNextOccurrence(DateTimeOffset.UtcNow, easternTimeZone);

If you passed a DateTime object, resulting time will be in UTC. If you used DateTimeOffset, resulting object will contain the correct offset, so don't forget to use it especially during DST transitions (see below).

If you just want to make all the calculations using local time, you'll have to use the DateTimeOffset class, because as I've said earlier, DateTime objects may be ambiguous during Summer time transitions. You can get the resulting local time, using the DateTimeOffset.DateTime property.

CronExpression expression = CronExpression.Parse("* * * * *");
DateTimeOffset? next = expression.GetNextOccurrence(DateTimeOffset.Now, TimeZoneInfo.Local);

var nextLocalTime = next?.DateTime;

If you want to specify seconds, use another overload of the Parse method and specify the CronFormat argument as below:

CronExpression expression = CronExpression.Parse("*/30 * * * * *", CronFormat.IncludeSeconds);
DateTime? next = expression.GetNextOccurrence(DateTime.UtcNow);

You can also get occurrences within a fixed date/time range using the GetOccurrences method. By default, the from argument will be included when matched, and to argument will be excluded. However, you can configure that behavior.

CronExpression expression = CronExpression.Parse("* * * * *");
IEnumerable<DateTime> occurrences = expression.GetOccurrences(
    DateTime.UtcNow,
    DateTime.UtcNow.AddYears(1),
    fromInclusive: true,
    toInclusive: false);

There are different overloads for this method to support DateTimeOffset arguments or time zones.

Cron expression is a mask to define fixed times, dates and intervals. The mask consists of second (optional), minute, hour, day-of-month, month and day-of-week fields. All of the fields allow you to specify multiple values, and any given date/time will satisfy the specified Cron expression, if all the fields contain a matching value.

                                       Allowed values    Allowed special characters   Comment

┌───────────── second (optional)       0-59              * , - /                      
│ ┌───────────── minute                0-59              * , - /                      
│ │ ┌───────────── hour                0-23              * , - /                      
│ │ │ ┌───────────── day of month      1-31              * , - / L W ?                
│ │ │ │ ┌───────────── month           1-12 or JAN-DEC   * , - /                      
│ │ │ │ │ ┌───────────── day of week   0-6  or SUN-SAT   * , - / # L ?                Both 0 and 7 means SUN
│ │ │ │ │ │
* * * * * *

In all fields you can use number, * to mark field as any value, - to specify ranges of values. Reversed ranges like 22-1(equivalent to 22,23,0,1,2) are also supported.

It's possible to define step combining / with *, numbers and ranges. For example, */5 in minute field describes every 5 minute and 1-15/3 in day-of-month field – every 3 days from the 1st to the 15th. Pay attention that */24 is just equivalent to 0,24,48 and */24 in minute field doesn't literally mean every 24 minutes it means every 0,24,48 minute.

Concatinate values and ranges by ,. Comma works like OR operator. So 3,5-11/3,12 is equivalent to 3,5,8,11,12.

In month and day-of-week fields, you can use names of months or days of weeks abbreviated to first three letters (Jan-Dec or Mon-Sun) instead of their numeric values. Full names like JANUARY or MONDAY aren't supported.

For day of week field, both 0 and 7 stays for Sunday, 1 for Monday.

Most expressions you can describe using base characters. If you want to deal with more complex cases like the last day of month or the 2nd Saturday use special characters:

L stands for "last". When used in the day-of-week field, it allows you to specify constructs such as the last Friday (5Lor FRIL). In the day-of-month field, it specifies the last day of the month.

W in day-of-month field is the nearest weekday. Use W with single value (not ranges, steps or *) to define the nearest weekday to the given day. In this case there are two base rules to determine occurrence: we should shift to the nearest weekday and can't shift to different month. Thus if given day is Saturday we shift to Friday, if it is Sunday we shift to Monday. But if given day is the 1st day of month (e.g. 0 0 1W * *) and it is Saturday we shift to the 3rd Monday, if given day is last day of month (0 0 31W 0 0) and it is Sunday we shift to that Friday. Mix L (optionaly with offset) and W characters to specify last weekday of month LW or more complex like L-5W.

# in day-of-week field allows to specify constructs such as second Saturday (6#2 or SAT#2).

? is synonym of *. It's supported but not obligatory, so 0 0 5 * ? is the same as 0 0 5 * *.

You can set both day-of-month and day-of-week, it allows you to specify constructs such as Friday the thirteenth. Thus 0 0 13 * 5 means at 00:00, Friday the thirteenth.

It differs from Unix crontab and Quartz cron implementations. Crontab handles it like OR operator: occurrence can happen in given day of month or given day of week. So 0 0 13 * 5 means at 00:00 AM, every friday or every the 13th of a month. Quartz doesn't allow specify both day-of-month and day-of-week.

A macro is a string starting with @ and representing a shortcut for simple cases like every day or every minute.

Cronos parser uses following case-insensitive grammar:

        cron ::= expression | macro
  expression ::= [second space] minute space hour space day-of-month space month space day-of-week
      second ::= field
      minute ::= field
        hour ::= field
day-of-month ::= '*' [step] | '?' [step] | lastday | value [ 'W' | range [list] ]
       month ::= field
 day-of-week ::= '*' [step] | '?' [step] | value [ dowspec | range [list] ]
       macro ::= '@every_second' | '@every_minute' | '@hourly' | '@daily' | '@midnight' | '@weekly' | '@monthly'|
                 '@yearly' | '@annually'
       field ::= '*' [step] | '?' [step] | value [range] [list]
        list ::= { ',' value [range] }
       range ::= '-' value [step] | [step]
        step ::= '/' number
       value ::= number | name
        name ::= month-name | dow-name
  month-name ::= 'JAN' | 'FEB' | 'MAR' | 'APR' | 'MAY' | 'JUN' | 'JUL' | 'AUG' | 'SEP' | 'OCT' | 'NOV' | 'DEC'
    dow-name ::= 'SUN' | 'MON' | 'TUE' | 'WED' | 'THU' | 'FRI' | 'SAT'
     dowspec ::= 'L' | '#' number
     lastday ::= 'L' ['-' number] ['W']
      number ::= digit | number digit
       space ::= ' ' | '\t'

Cronos is the only library to handle daylight saving time transitions in intuitive way with the same behavior as Vixie Cron (utility for *nix systems). During a spring transition, we don't skip occurrences scheduled to invalid time during. In an autumn transition we don't get duplicate occurrences for daily expressions, and don't skip interval expressions when the local time is ambiguous.

During the transition to Summer time, the clock is moved forward, for example the next minute after 01:59 AM is 03:00 AM. So any daily Cron expression that should match 02:30 AM, points to an invalid time. It doesn't exist, and can't be mapped to UTC.

Cronos adjusts the next occurrence to the next valid time in these cases. If you use Cron to schedule jobs, you may have shorter or longer intervals between runs when this happen, but you'll not lose your jobs:

"30 02 * * *" (every day at 02:30 AM)

Mar 13, 02:30 +03:00 – run
Mar 14, 03:00 +04:00 – run (adjusted)
Mar 15, 02:30 +04:00 – run

When Daylight Saving Time ends you set the clocks backward so there is duration which repeats twice. For example, after 01:59 AM you get 01:00 AM again, so the interval between 01:00 AM to 02:00 AM (excluding) is ambiguous, and can be mapped to multiple UTC offsets.

We don't want to have multiple occurrences of daily expressions during this transition, but at the same time we want to schedule interval expressions as usually, without skipping them. So we have different behavior for different Cron expressions.

Cron expression is interval based whose second, minute or hour field contains *, ranges or steps, e.g. 30 * * * * (hour field), * 1 * * * (minute field), 0,5 0/10 1 * * *. In this case there are expectations that occurrences should happen periodically during the day and this rule can't be broken by time transitions. Thus for interval based expressions occurrences will be before and after clock shifts.

Consider */30 * * * * interval expression. It should occur every 30 minutes no matter what.

Nov 08, 00:30 +04:00 – run
Nov 08, 01:00 +04:00 – run
Nov 08, 01:30 +04:00 – run
Nov 08, 01:00 +03:00 – run
Nov 08, 01:30 +03:00 – run
Nov 08, 02:00 +03:00 – run

Cron expression is non-interval based whose second, minute or hour field does not contain *, ranges or steps, e.g. 0 30 1 * * * or 0 0,45 1,2 * * *. We expect they occur given number of times per day. Thus for non-interval expressions occurrences will be just before clock shifts.

Consider 30 1 * * * non-interval expression. It should occur once a day no matter what.

Nov 07, 01:30 +04:00 – run
Nov 08, 01:30 +04:00 – run
Nov 08, 01:30 +03:00 – skip
Nov 09, 01:30 +03:00 – run

Since BenchmarkDotNet project appeared, it's hard to ignore the performance. We tried hard to make Cronos not only feature-rich, but also really fast when parsing expressions and calculating next occurrences. As a result, Cronos is faster more than in an order of magnitude than alternative libraries, here is a small comparison:

Cronos Method                               |           Mean |        StdDev
------------------------------------------- | -------------- | -------------
CronExpression.Parse("* * * * *")           |     30.8473 ns |     0.0515 ns
CronExpression.Parse("*/10 12-20 ? DEC 3")  |     81.5010 ns |     0.0924 ns
Simple.GetNextOccurrence(DateTime.UtcNow)   |    123.4712 ns |     0.5928 ns
Complex.GetNextOccurrence(DateTime.UtcNow)  |    212.0422 ns |     0.3997 ns

NCrontab Method                             |           Mean |        StdDev
------------------------------------------- | -------------- | -------------
CrontabSchedule.Parse("* * * * *")          |  1,813.7313 ns |     3.3718 ns
CrontabSchedule.Parse("*/10 12-20 * DEC 3") |  3,174.3625 ns |     6.8522 ns
Simple.GetNextOccurrence(DateTime.UtcNow)   |    147.7866 ns |     0.1689 ns
Complex.GetNextOccurrence(DateTime.UtcNow)  |  1,001.3253 ns |     1.6205 ns

Quartz Method                               |           Mean |        StdDev
------------------------------------------- | -------------- | -------------
new CronExpression("* * * * * ?")           | 48,157.7744 ns | 1,417.3101 ns
new CronExpression("* */10 12-20 ? DEC 3")  | 33,731.9992 ns |    38.3192 ns
Simple.GetTimeAfter(DateTimeOffset.Now)     |  1,416.9867 ns |     1.2784 ns
Complex.GetTimeAfter(DateTimeOffset.Now)    |  6,573.0269 ns |     7.9192 ns

Copyright © 2017 Hangfire OÜ. Cronos is licensed under The MIT License (MIT).