See date command.

Resources

See How To Format Date And Time In Linux, MacOS, And Bash?.

Unix timestamp

Use a built-in variable for current time in seconds.

$ echo $EPOCHSECONDS
1614012035

Or as a float.

$ echo $EPOCHREALTIME
1614012035.3449261189

Or use the date command with a format.

$ date '+%s'
1614012035