Most of the date functions return the timestamp in milliseconds since the Unix epoch. You could store that value directly in a text or numeric variable.
Since ‘epoch time’ is the seconds or milliseconds since the ‘unix epoch’ (January 1st, 1970 at 00:00:00 UTC), keep in mind that a particular time of day (eg. “6:00 AM”) isn’t something that can be directly expressed in ‘epoch time’. A specific time on a specific day is something that could be expressed in ‘epoch time’ though… so I suppose it depends on exactly what you’re attempting to do.
If you have a particular time stored as a string, an approach was discussed in some detail above, so I would recommend reading that post: Math / Date Expressions! - #5 by josh