It’s egregious that Apple still doesn’t provide any skip functions in Quicktime. I guess that’s why most people I know — myself included — install IINA or VLC asap.
I would love to be able to skip forward/back by 5sec with a keyboard shortcut in DT built-in video player, which I assume is a derivative of Apple’s unloved player.
Does J-K-L work in the player?
That’s a somewhat standard playback in the QuickTime Player and a lot of editing software.
One L is 1x speed forward
LL is 2x forward
LLL is 5x forward
K is pause
J is 1x speed backward or -1 from the forward speed
KL together is slomo forward
JK together is slomo backward
This script does it.
-- Skip video
property theSeconds : 5
property forward : true
tell application id "DNtp"
try
set theRecord to content record of think window 1
if theRecord = missing value then error "Please open a video record"
set theRecord_Type to (type of theRecord) as string
if theRecord_Type ≠ "quicktime" then error "Please open a video record"
set theTab_CurrentTime to current time of current tab of think window 1
if forward then
set theTime to (theTab_CurrentTime + theSeconds)
set theRecord_Duration to duration of theRecord
if theTime > theRecord_Duration then
set theTime to theRecord_Duration
end if
else
set theTime to (theTab_CurrentTime - theSeconds)
end if
do shell script "open " & (reference URL of theRecord & "?time=" & theTime)
on error error_message number error_number
if the error_number is not -128 then display alert "DEVONthink" message error_message as warning
return
end try
end tell
@cgrunenberg Trying to set the current time directly yields an AppleScript error.
Shift-Left/Right jumps 15 seconds back/forward. There’s even a hidden preference to customize the interval. No need for VLC 
An issue of the script suite, the current time propery should be declared as read-only as it’s obviously not possible to change it.
Anyway, the next release will support this.
Is there a list of shortcuts for the video player? I’m wondering if there are any other useful ones like changing the playback speed, as that is buried in the media bar menu.