# Expression tag - dateDiff error **URL:** \ **Category:** Ignition\ **Tags:** perspective\ **Created:** [March 19, 2024, 11:11pm UTC](https://forum.inductiveautomation.com/t/expression-tag-datediff-error/87599 "2024-03-19T23:11:38Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![SCADA.Guy](https://avatars.discourse-cdn.com/v4/letter/s/838e76/32.png) [@SCADA.Guy](https://forum.inductiveautomation.com/u/SCADA.Guy)\ **Post date:** [March 19, 2024, 11:11pm UTC](https://forum.inductiveautomation.com/t/expression-tag-datediff-error/87599/1 "2024-03-19T23:11:38Z")
Hello! I've created an expression tag to keep track of the time once an OPC tag stops updating. However, while the OPC tag is continuously updating it returns a non-zero value larger than it should. Once the OPC tag does stop, it then starts adding time to that non-zero value. This leads to the expression tag returning an incorrect time even when the OPC tag is stopped. Is there a better/different way I can do this? The expression tag is written as: dateDiff({[.]tag.LastChange},now(),"seconds") ---
**Author:** ![Transistor](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/transistor/32/3380_2.png) [@Transistor](https://forum.inductiveautomation.com/u/Transistor)\ **Post date:** [March 19, 2024, 11:22pm UTC](https://forum.inductiveautomation.com/t/expression-tag-datediff-error/87599/2 "2024-03-19T23:22:09Z")
Some clarification is needed: > I've created an expression tag to keep track of the time once an OPC tag stops updating. Do you mean the expression tag should report the time _since the most recent timestamp_? > However, while the OPC tag is continuously updating it returns a non-zero value larger than it should. Give an example. There will be at least a few milliseconds difference. What is the scan class (update rate) on the OPC tag? > Once the OPC tag does stop, it then starts adding time to that non-zero value. This part isn't clear. > This leads to the expression tag returning an incorrect time even when the OPC tag is stopped. Is there a better/different way I can do this? Do you mean "returning an incorrect _elapsed_ time"? > `dateDiff({[.]tag.LastChange}, now(), "seconds")` Be aware of the `now()` function parameter. `now(0)`: executes once at startup or view load. `now()`: executes at the default rate - usually 1000 ms. `now(250)`: executes every 250 ms. Please be clear what you are trying to achieve. e.g., "The expression tag should report the number of seconds since the most recent OPC tag timestamp." ---
**Author:** ![SCADA.Guy](https://avatars.discourse-cdn.com/v4/letter/s/838e76/32.png) [@SCADA.Guy](https://forum.inductiveautomation.com/u/SCADA.Guy)\ **Post date:** [March 19, 2024, 11:43pm UTC](https://forum.inductiveautomation.com/t/expression-tag-datediff-error/87599/3 "2024-03-19T23:43:19Z")
> I've created an expression tag to keep track of the time once an OPC tag stops updating. The OPC tag reaches a value of 10000, after that it does not continue. I want to keep track of the time since it reached that value. > However, while the OPC tag is continuously updating it returns a non-zero value larger than it should. While the OPC tag is somewhere between 1-10000, the expression tag returns a value of anywhere between 100-500. It varies, but let's say it's 500. It will remain at 500 until the tag reaches 10000. > Once the OPC tag does stop, it then starts adding time to that non-zero value. Once the OPC tag does stop (when it reaches 10000). The expression tag will then start adding to that 500. It will return 501....502....503.... updating every second. > This leads to the expression tag returning an incorrect time even when the OPC tag is stopped. Is there a better/different way I can do this? Yes, the incorrect elapsed time. The expression tag should now be returning 1....2....3.... but is instead adding that 500 onto it. ---
**Author:** ![pturmel](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/pturmel/32/3295_2.png) [@pturmel](https://forum.inductiveautomation.com/u/pturmel)\ **Post date:** [March 19, 2024, 11:48pm UTC](https://forum.inductiveautomation.com/t/expression-tag-datediff-error/87599/4 "2024-03-19T23:48:10Z")
I'm going to guess there's a clock error between the source of that OPC tag and the gateway. ---
**Author:** ![Kevin.Herron](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/kevin.herron/32/7259_2.png) [@Kevin.Herron](https://forum.inductiveautomation.com/u/Kevin.Herron)\ **Post date:** [March 19, 2024, 11:51pm UTC](https://forum.inductiveautomation.com/t/expression-tag-datediff-error/87599/5 "2024-03-19T23:51:58Z")
> [@SCADA.Guy](#): > > The OPC tag reaches a value of 10000, after that it does not continue. I want to keep track of the time since it reached that value. You know the value it stops at, so it would be easier to reframe this. Create a Date tag that you write the current date to, from a tag event script on the OPC tag when value == 10000. When it's not 10000 write a null or some other sentinel value. You can calculate how long since it's been 10000 any time you need from that tag as long as it's not the null/sentinel value. ---
**Author:** ![SCADA.Guy](https://avatars.discourse-cdn.com/v4/letter/s/838e76/32.png) [@SCADA.Guy](https://forum.inductiveautomation.com/u/SCADA.Guy)\ **Post date:** [March 20, 2024, 12:13am UTC](https://forum.inductiveautomation.com/t/expression-tag-datediff-error/87599/6 "2024-03-20T00:13:39Z")
Thanks! That makes sense. I'm quite new to this though, and have a couple of questions. > Create a Date tag that you write the current date to I'm not sure what type of tag this would be? As in an expression, memory, etc. I don't see Date tag as a type. > from a tag event script on the OPC tag when value == 10000 Can you give me an idea of what that tag event script would look like? I would like it to write to the date tag every second once the value == 10000 and would also like the elapsed time to be in seconds.