Dev Brains AI
AIDevTools

Cron Expressions for AWS EventBridge and Lambda

AWS EventBridge (formerly CloudWatch Events) lets you schedule Lambda functions, Step Functions, ECS tasks, and other targets using either a rate expression or a cron expression. AWS cron syntax is similar to standard Unix cron but has some important differences that catch many developers off guard.

AWS cron syntax — 6 fields

AWS EventBridge uses 6 fields, not 5:

cron(minute hour day-of-month month day-of-week year)
      ↑     ↑         ↑          ↑        ↑       ↑
      0-59  0-23     1-31       1-12     1-7    1970-2199

Key difference from standard cron: AWS uses 1–7 for days (1=Sunday) instead of 0–6 (0=Sunday), and adds a year field.

The ? wildcard — day-of-month XOR day-of-week

AWS requires that exactly one of day-of-month and day-of-week be * or ?. If you specify a day-of-week, day-of-month must be ? and vice versa.

# Every Monday at 9 AM UTC
cron(0 9 ? * 2 *)     # day-of-month = ? because day-of-week is set

# First of every month at midnight UTC
cron(0 0 1 * ? *)     # day-of-week = ? because day-of-month is set

# Every day at 6 PM UTC
cron(0 18 * * ? *)    # use ? for day-of-week when not constrained

Practical examples

  • cron(0 12 * * ? *) — every day at noon UTC
  • cron(0 2 ? * 2 *) — every Monday at 2 AM UTC
  • cron(*/5 * * * ? *) — every 5 minutes
  • cron(0 8 1 * ? *) — first of every month at 8 AM UTC
  • cron(0 9 ? * 2-6 *) — weekdays (Mon–Fri) at 9 AM UTC
  • cron(0 0 ? 1,4,7,10 1 *) — first Sunday of Jan, Apr, Jul, Oct

Rate expressions — simpler for intervals

For simple interval schedules, AWS rate expressions are easier than cron:

rate(5 minutes)    # every 5 minutes
rate(1 hour)       # every hour
rate(7 days)       # every 7 days
# Note: singular for value=1 (rate(1 minute) not rate(1 minutes))

Scheduling in Terraform

resource "aws_cloudwatch_event_rule" "daily_job" {
  name                = "daily-lambda-trigger"
  schedule_expression = "cron(0 9 ? * 2-6 *)"  # weekdays 9 AM UTC
}

resource "aws_cloudwatch_event_target" "lambda_target" {
  rule = aws_cloudwatch_event_rule.daily_job.name
  arn  = aws_lambda_function.my_function.arn
}

Timezone handling

EventBridge cron always runs in UTC unless you use the newer EventBridge Scheduler (not CloudWatch Events), which supports specifying a timezone directly in the schedule. To run a Lambda at 9 AM IST (UTC+5:30), you would set the cron time to 3:30 UTC in standard EventBridge rules:

# 9:00 AM IST = 3:30 AM UTC
cron(30 3 ? * 2-6 *)

Generate your cron expression

Use the Dev Brains AI Cron Generator to build your base cron expression from plain English, then adjust it to AWS format by adding the year field and using ? where required.