@Madsheogh
Try to put your formula modification of:
Set(_firstDayInView,DateAdd(_firstDayOfMonth,-(Mod(Weekday(_firstDayOfMonth, StartOfWeek.Monday)-2,7)+1),Days))
in all these places inside the Calendar component, replacing the appropriate line of the formula of course:
tmrInitialize, in the OnTimerStart property
btnNextMonth, in the OnSelect property
btnPrevMonth, in the OnSelect property
btnDebugInit, in the OnSelect property
Let me know if it worked.
I might update this post with any more places in the app I find where this replacement may need to be done.
I suspect there are more than the 4 places I initially found of: tmrInitialize OnTimerStart and btnNextMonth btnPrevMonth btnDebugInit OnChange - if I find it I might update this post.
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I found more, and I might update this post again when I find them:
ddMonth, in the OnChange property
ddYear, in the OnChange property
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I think those 4 places:
tmrInitialize OnTimerStart and btnNextMonth btnPrevMonth btnDebugInit OnChange
and 2 more places:
ddMonth, ddYear OnChange
are all of them, for a total of 6 formula replacements.

I think that should be all of them,
but if you find any others make sure to change them there too, and be sure to post of any others you found than the ones I gave.
Check if making the careful formula replacement on all 6 of the above places resolves the issue for you @Madsheogh