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Per comments...
defaults read com.apple.notificationcenterui
gives

{
    TodayView =     {
        NoContent =         (
            "com.apple.reminders.RemindersNC"
        );
        keyWidget = "com.apple.ncplugin.calculator";
        order =         (
            "com.apple.reminders.RemindersNC",
            "com.apple.iTunes.today.TodayExtension",
            "com.apple.iCal.CalendarNC",
            "com.apple.ncplugin.weather",
            "com.parallels.desktop.console.Resource-Monitor",
            "com.apple.ncplugin.stocks"
        );
        preferences =         {
            "com.apple.nc.disclosures" =             {
                enabled = 1;
            };
            "com.apple.nc.today.date" =             {
                enabled = 1;
            };
            "com.apple.nc.today.dnd" =             {
                enabled = 1;
            };
            "com.apple.nc.today.summary" =             {
                enabled = 1;
            };
            "com.apple.nc.tomorrow.summary" =             {
                enabled = 1;
            };
        };
    };
    bannerTime = 5;
    "last-messagetrace-stamp" = "551957496.077987";

so they're missing there. Does that mean there's a defaults write to put them back?

I only just noticed this today - Calculator is gone.

Is this an "improvement" with 10.13.5 which I installed just the other day, or did I break something?

I did notice Stocks appear in there as if by magic the other day - something I'm never ever going to need, but I hadn't spotted Calculator's disappearance until now.

Checked my partner's machine - same OS, very similar machine - lot & lots of widgets...

So, maybe the question becomes, 'where do they live & why can't my machine find them?'

I only just noticed this today - Calculator is gone.

Is this an "improvement" with 10.13.5 which I installed just the other day, or did I break something?

I did notice Stocks appear in there as if by magic the other day - something I'm never ever going to need, but I hadn't spotted Calculator's disappearance until now.

Checked my partner's machine - same OS, very similar machine - lot & lots of widgets...

So, maybe the question becomes, 'where do they live & why can't my machine find them?'

Late edit:
I've found the calculator app, in
/System/Library/Frameworks/NotificationCenter.framework/PlugIns/com.apple.ncplugin.calculator.appex
so now I need to figure out a way to get the Notification Centre to recognise it again.

I only just noticed this today - Calculator is gone.

Is this an "improvement" with 10.13.5 which I installed just the other day, or did I break something?

I did notice Stocks appear in there as if by magic the other day - something I'm never ever going to need, but I hadn't spotted Calculator's disappearance until now.

I only just noticed this today - Calculator is gone.

Is this an "improvement" with 10.13.5 which I installed just the other day, or did I break something?

I did notice Stocks appear in there as if by magic the other day - something I'm never ever going to need, but I hadn't spotted Calculator's disappearance until now.

Checked my partner's machine - same OS, very similar machine - lot & lots of widgets...

So, maybe the question becomes, 'where do they live & why can't my machine find them?'