Date arithmetic in conditionals for a SharePoint workflow

Question

I'm working on a SharePoint list that collects items submitted via a form. The items have various statuses, and I have workflows set up that send appropriate emails when these statuses change (e.g., when the item creator changes their status from "draft" to "ready for review," an email is sent to the approver that says "hey you need to go review this now").

The items in the list need to be revised and re-approved a year after they are approved. My client has asked for automated reminders to be sent three months prior to this drop-dead date, saying "hey, the approval on your thing is gonna expire soon, please review it and resubmit it for review."

My plan to implement this is to use a workflow that runs once a day, and sends an email if and only if:

• The item status is "approved."
• The current date is at least nine months later than the last-modified date.

However, I don't see a way to do the kind of simple arithmetic needed for the second bullet point in a workflow conditional. It's easy to say "send it if the current date equals the last-modified date," but not to day "send it if the current date is greater than or equal to the last-modified date plus nine months."

Is this achievable? OOTB solutions in SharePoint Designer are preferable, but if that's a no-go, I'm open to other options.

Answer 1

This might seem weird, but here's how I got it to work:

I created a workflow variable to store the calculation of the modified date. I had two options for how to return the modified date; Double, or number of seconds since 1/1/1900. I chose the number of seconds. In my variable, I store the result of Modified Date (returned as number of seconds), plus 23,328,000 (which is the number of seconds in 270 days, or about 9 months). If I then set a Date field to the value of the variable, it converts it into an actual date. For example, if my item is modified today, the calculation returns April 26th, 2019. Here is a sample of the test workflow:

And here is the list item after the workflow has run (ignore the extra columns... this is a list I use for testing SE answers):

Let me know if this can help you or not! :)