Select Data Conversion action, and then click Convert Date & Time Format.

Choose the date, time, or output that you want to convert to UNIXTIME.
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Select the format of the date or time you are converting based on how it was originally set.

For the Target Date & Time Format, select 10-digit number (UNIXTIME) format.

This will convert the date into UNIXTIME, which is represented as a 10-digit numeric string (see example below).

After testing, the output will show a string of numbers - this is the UNIXTIME.

Use this output in your flow.