For those using Slack on MacOS Sonoma, there is a known issue that causes time to be displayed incorrectly, at least here in Canberra during Daylight Savings Time (AEDT). One workaround is to launch Slack from the command line as follows:
codesign -dr - /Applications/Slack.app
TZ=Country/City open -b com.tinyspeck.slackmacgap
where you should replace Country/City with your own standard location for time zones, e.g. Australia/Sydney.