Hi Nick
I have reproduced the error in a vanilla WordPress installation.
This has latest WP, latest Simple Calendar, uses WP TwentySeventeen theme, no other plugins installed.
I have put on 3 posts to show the problem.
http://development.ahswebservices.com/
The first shows the grid view month display error.
Calendar settings; Start = This month, Earliest event = same as start
The second shows the grid view with
Calendar settings; Start = Today, Earliest event = same as start
The third shows the grid view with a bit of a cludge – at least it gets the display of current month correct, but it still has issue if you click to go to next month, it jumps to August (misses July!), but you can then go back to July, then June OK.
Calendar settings; Start = 1 month after current, Earliest event = 1 month before current
I think the underlying issue is to do with the Time Zone setting.
I notice that in the Advanced settings for the calendar that the Display Time Format preview shows the time as London GMT, and not local time as in London BST.
The WordPress settings is set to London, but it picks up the daylight savings OK:
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Timezone London
Choose either a city in the same timezone as you or a UTC timezone offset.
Universal time (GMT) is 2017-06-19 14:25:41. Local time is 2017-06-19 15:25:41.
This timezone is currently in daylight saving time.
Standard time begins on: 29th October 2017 2:00 am.
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I know that the Simple Calendar WAS working OK after the installation of your version 3.1.8 in October 18th., over the Winter when London was in GMT.
My previous logged call with you with the issue was probably before end of October 2016, when we were in London BST, and also when it changed to London GMT at the end of October 2016.
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3.1.8 – October 18, 2016
Fix: Multi-day events should not show duplicate events in one day in some time zones.
Fix: List view heading dates corrected for some time zones.
Fix: Grid view starting month corrected for some time zones.
Fix: Remove duplicate “Event source default” option when add-on enabled.
Tweak: Set “Expand multi-day events” default value to “No, display only on first day of event”.
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Since the update to 3.1.9 could it be that Simple Calendar is not picking up the fix to the daylight savings time?
I have tried to re-install 3.1.8, by deactivating + delete Simple Calendar plugin, and then using the download of your 3.1.8 zip file, but this seems to screw up completely the website and it requires a full restore to get it back again. So not sure if there is another way to get back to 3.1.8?