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Calculating difference between two date custom fields in Trello

I am looking if there is a way to calculate the difference between two date custom fields using Butler? Ex: I have 'Start Date' and 'Completion Date' as 2 custom fields and if I want to know the duration of the task, want to write a Butler rule which will calculate the difference of these 2 dates and return in another custom field of 'Task time'. Can someone help?

3 answers

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richard hoare
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Feb 01, 2024

hey folks, not sure if anyone got any further with this, but i came up with a fix last night after a few attempts.. here's the basic gist..


you'll need three custome fields: 1. Start Date (presumably populated earlier on in board); 2. Completion Date (in my case, populated when you move the card to list "Completed"); 3. Days (which is numeric field where the following string comes in. 


when a card is added to list "Completed", set date custom field "Completion Date" to now, set custom field "Days" to "({{%Completion Date}~X} - {{%Start Date}~X}) / 86400"


what's happening... the Completion Date and Start Date are converted to Unix time code (using "~X") - they're then subtracted from one another which gives an absolute value in seconds. that calculation is then divided by number of seconds in a day (86400) - but the key part to this was placing the initial equation in standard brackets (wrapped around the curly brackets) in order to ensure the subtracted total figure is the figure which is being divided by 86400. 


hope this helps someone!




I'd like to know the answer as well. I have the same message. Thanks!

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Vero Rivas
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Apr 29, 2021

Hi @Manjula Madineedi 

   Look at this post, maybe can help you



Thanks for sharing @Vero Rivas . But that doesn't work for me. Here is the rule I wrote:

when a card is moved into list "Completed", set custom field "* Hours Task Time" to "{{%Completion Date}-{%In Progress Start}}"

When I checked the Command log of the rule, it had this message -- "NaN" is not a valid numeric value.

I am not sure what's wrong in the above rule..

I'd like to know the answer to this as well. I have the same custom field set up with start date. Trello seems to have updated the calendar to allow start and end dates. That's the only way I've been able to get the timeframe between start and end. 

Like PROYECTOS DC SPAIN likes this

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