OnCalendar entrys can be specified multiple times in a systemd timer, to
make more complex scheduling possible.
Tested by manually checking the timer generated by the following:
systemd = {
services.huhu = {
description = "meh";
wantedBy = [ "default.target" ];
serviceConfig.ExecStart = "/bin/sh -c 'printf HUHU!'";
startAt = [ "*:*:0/30" "*:0/1:15" ];
};
};
It prints HUHU to the log at seconds 0, 15 and 30 of each minute.