[TSK-1085] Update months passed calculation

[wazsone]

Contribution checklist

Brief description

Related issues

https://front.hc.engineering/workbench/platform/tracker/TSK-1085

Checklist

• - UI test added to added/changed functionality?
• - Screenshot is added to PR if applicable ?
• - Does the code work? Check whether function and logic are correct.
• - Does Changelog.md is updated with changes?
• - Does the translations are up to date?
• - Does it well tested?
• - Tested for Chrome.
• - Tested for Safari.
• - Go through the changed code looking for typos, TODOs, commented LOCs, debugging pieces of code, etc.
• - Rebase your branch onto master and upstream branch
• - Is there any redundant or duplicate code?
• - Are required links are linked to PR?
• - Does new code is well documented ?