My guess is that the datetime values are not considered to be monotonically increasing because they span multiple years.
Try this as a work-around —
LD = load('19year.mat');
elev = LD.elev;
timing = LD.timing;
[pks,plocs] = findpeaks(elev);
[vys,vlocs] = findpeaks(-elev);
figure
plot(timing, elev, 'DisplayName','Data')
hold on
plot(timing(plocs), pks, '^r', 'MarkerSize',2.5, 'DisplayName','Peaks')
plot(timing(vlocs), -vys, 'vg', 'MarkerSize',2.5, 'DisplayName','Valleys')
hold off
grid
xlabel('timing')
ylabel('elev')
legend('Location','best')
figure
plot(timing, elev, 'DisplayName','Data')
hold on
plot(timing(plocs), pks, '^r', 'DisplayName','Peaks')
plot(timing(vlocs), -vys, 'vg', 'DisplayName','Valleys')
hold off
grid
xlabel('timing')
ylabel('elev')
legend('Location','best')
xlim([timing(1) timing(500)])
.
