I found this issue(docker/cli#2071) in the cli repository but tracked the source of the issue here. I'm just going to copy/paste the original issue description as its still relevant.

Description

The documentation for docker system prune on 19.03 states that the until filter supports timestamps, but it doesn't work.

Steps to reproduce the issue:

1. Run `docker system prune --force --all --filter "until=2006-01-02T15:04:05"`

Describe the results you received:

Error response from daemon: failed to prune build cache: "until" filter expects a duration (e.g., '24h'): time: unknown unit - in duration 2006-01-02T15:04:05

Describe the results you expected:
The prune to work with the timestamp.

Output of docker version:

Client: Docker Engine - Community
 Version:           19.03.1
 API version:       1.40
 Go version:        go1.12.5
 Git commit:        74b1e89
 Built:             Thu Jul 25 21:22:03 2019
 OS/Arch:           linux/amd64
 Experimental:      false

Server: Docker Engine - Community
 Engine:
  Version:          19.03.1
  API version:      1.40 (minimum version 1.12)
  Go version:       go1.12.5
  Git commit:       74b1e89
  Built:            Thu Jul 25 21:20:35 2019
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          1.2.6
  GitCommit:        894b81a4b802e4eb2a91d1ce216b8817763c29fb
 runc:
  Version:          1.0.0-rc8
  GitCommit:        425e105d5a03fabd737a126ad93d62a9eeede87f
 docker-init:
  Version:          0.18.0
  GitCommit:        fec3683

Output of docker info:

Client:
 Debug Mode: false

Server:
 Containers: 42
  Running: 0
  Paused: 0
  Stopped: 42
 Images: 144
 Server Version: 19.03.1
 Storage Driver: aufs
  Root Dir: /var/lib/docker/aufs
  Backing Filesystem: extfs
  Dirs: 611
  Dirperm1 Supported: true
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
 Swarm: inactive
 Runtimes: runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 894b81a4b802e4eb2a91d1ce216b8817763c29fb
 runc version: 425e105d5a03fabd737a126ad93d62a9eeede87f
 init version: fec3683
 Security Options:
  apparmor
  seccomp
   Profile: default
 Kernel Version: 4.19.0-5-amd64
 Operating System: Debian GNU/Linux 10 (buster)
 OSType: linux
 Architecture: x86_64
 CPUs: 12
 Total Memory: 15.62GiB
 Name: server123
 ID: ZSS4:GQ3D:IWCX:FTWI:WBPU:7X4W:AMW7:6JWJ:ISPK:PSBG:HZXH:J6MU
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Registry: https://index.docker.io/v1/
 Labels:
 Experimental: false
 Insecure Registries:
  127.0.0.0/8
 Live Restore Enabled: false

WARNING: the aufs storage-driver is deprecated, and will be removed in a future release.