nixos/postgresqlBackup: allow defining multiple times to start at
nixos/postgresqlBackup: allow defining multiple times to start at

Or … none! Because forcing a string always results in an OnCalender=
setting, but an empty string leads to an empty value.

>  postgresqlBackup-hass.timer: Timer unit lacks value setting. Refusing.

or

> postgresqlBackup-miniflux.timer: Cannot add dependency job, ignoring: Unit postgresqlBackup-miniflux.timer has a bad unit file setting.

I require the postgresqlBackup in my borgbackup unit, so I don't
strictly need the timer and could previously set it to an empty list.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs