Unix time is an integer that represents the number of seconds that have passed since January 1st, 1970 UTC (Coordinated Universal Time). It's a way to track time as a running total of seconds.
Here's a handy table with some conversions:
|What we use||Time in seconds|
|1 Hour||3600 Seconds|
|1 Day||86400 Seconds|
|1 Week||604800 Seconds|
|1 Month (30.44 days)||2629743 Seconds|
|1 Year (365.24 days)||31556926 Seconds|
You can use a number of online calculators to convert human-readable dates into Unix time. Unix Time Stamp is a good one.