Pick a moment in time and copy the code into any Discord message. Discord renders it in each person's own timezone, so "in 3 hours" and the exact date/time stay correct for everyone.
<t:1784051110:f><t:1784051110:F><t:1784051110:d><t:1784051110:D><t:1784051110:t><t:1784051110:T><t:1784051110:R>Tip: <t:1784051110> (no letter) is the same as the fformat. Previews above use your device's timezone; each Discord user sees their own.