startofday

This page explains how to use the startofday function in APL.

Use the startofday function in APL to round a datetime value down to the start of the day. The function returns midnight (00:00:00) for the date that contains the given datetime value. You can optionally shift the result by a specified number of days using the offset parameter.

You can use startofday to bin events into daily buckets for aggregation, reporting, and trend analysis across log, trace, and security datasets.

Use it when you want to:

Group events by day for daily summaries and dashboards.
Align timestamps to day boundaries for consistent aggregation.
Compare metrics across different days.

Usage

Syntax

APL

startofday(datetime [, offset])

Parameters

Name	Type	Description
datetime	`datetime`	The input datetime value.
offset	`long`	Optional: The number of days to offset from the input datetime. Default is `0`.

Returns

A datetime representing the start of the day (00:00:00) for the given date value, shifted by the offset if specified.

Use case examples

Count requests per day to identify daily traffic patterns.

Query

APLRun in Playground

['sample-http-logs']
| extend day_start = startofday(_time)
| summarize request_count = count() by day_start
| sort by day_start asc

Output

day_start	request_count
2025-01-13T00:00:00Z	1523
2025-01-14T00:00:00Z	1687
2025-01-15T00:00:00Z	1445

This query bins each HTTP request to the start of its day and counts the total requests per day.

Calculate the daily average span duration for each service.

Query

APLRun in Playground

['otel-demo-traces']
| extend day_start = startofday(_time)
| summarize avg_duration = avg(duration) by day_start, ['service.name']
| sort by day_start asc

Output

day_start	service.name	avg_duration
2025-01-13T00:00:00Z	frontend	00:00:01.2340000
2025-01-14T00:00:00Z	frontend	00:00:01.1750000
2025-01-15T00:00:00Z	frontend	00:00:01.2890000

This query groups trace spans by day and service, then calculates the average span duration for each combination.

Track daily error counts to identify days with unusual server error activity.

Query

APLRun in Playground

['sample-http-logs']
| where toint(status) >= 500
| extend day_start = startofday(_time)
| summarize error_count = count() by day_start
| sort by day_start asc

Output

day_start	error_count
2025-01-13T00:00:00Z	12
2025-01-14T00:00:00Z	27
2025-01-15T00:00:00Z	8

This query filters for server errors and counts them per day to reveal daily error patterns.

endofday: Returns the end of the day for a datetime value.
startofweek: Returns the start of the week for a datetime value.
startofmonth: Returns the start of the month for a datetime value.
startofyear: Returns the start of the year for a datetime value.
bin: Rounds values down to a fixed-size bin, useful for grouping timestamps.

startofday

Usage

Syntax

Parameters

Returns

Use case examples

List of related functions

Other query languages