PI 101, Module 4: Time and the PI System

Discussion created by Pablo Benvenuto on Sep 28, 2012

Welcome back! You are just "in time" for PI training :) Get it? "in time" Ok, it was a bad joke, but this training in PI Time is not bad at all. Enjoy!

PI time abbreviations

When specifying PI time, you can use specific abbreviations that represent time units and reference times.

To specify time units, you can specify the abbreviation, the full time unit, or the plural version of the time unit, such as s, second, or seconds. You must include a valid value with any time unit. If specifying seconds, minutes, or hours, you can specify a fractional value. You cannot specify fractional values for other time units.

PI time expressions

PI time expressions can include a reference time and a time offset, indicated by a direction (either + or -) and a time unit with a value. PI time expressions might include:
Only a reference time, such as y
Only a time offset, such as +3h

A reference time with a time offset, such as y+3h
A reference time can be a fixed time, such as 24-aug-2012 09:50:00, or a valid reference-time abbreviation, such as t.
You can only include one time offset in an expression. Including multiple offsets can lead to unpredictable results. For example, the following time expressions are not valid:
*+1d+4h
t-1d+12h

Time-stamp specification

To specify inputs for time stamps, you can enter time expressions that contain:

Fixed times: A fixed time always represents the same time, regardless of the field or the current time.

Reference-time abbreviations: A reference-time abbreviation represents a time relative to the current time.

Reference-time abbreviations or fixed times with a time offset: When included with a fixed time or a reference-time abbreviation, a time offset adds or subtracts from the specified time.

Time offsets: Entered alone in a time field, time offsets specify a time relative to an implied reference time. The implied reference time depends on the field where you enter the expression:

• For a start time, the reference time is the current clock time.
• For an end time, the reference time is the start time.
• For a single time stamp, the reference time is the current clock time.

How does the PI System Adjust for timezones and DST?

The short answer is, we don’t! When we collect data we convert it to UTC (Universal Coordinated Time), or what used to be called Greenwich Mean Time (GMT). This means that each day has exactly 24 hours. Any adjustments for time, such as timezone or Daylight Saving Time (DST), are made by the local machine clock of the user looking at the data.

So once a year the day will look like it has 23 hours and another 25, but the PI Server never knows anything other than 24 hour days. Also, because the clients and PI Server know what timezone they are in, the data can be viewed in either Server Time or Client Time. This is determined by a setting in the client tool.

Review Questions

Activity

Do you have any PI ProcessBook displays or PI DataLink reports at work? Examine the time or the time ranges displayed on the trends or the data arrays. After reading this module, can you easily convert PI Time notation to a standard time notation?

You can now go to Module 5 or back to the Workshop Outline.

MORE LEARNING OPTIONS

You might find these videos on PI Time useful.