Seconds since when?

You need some sort of base- (or zero-) time to convert a date to anything meaningful.

Choices include (among others):

• Epoch time (specify whose epoch AND a Time Zone)
• Your birth date and time (adjust for TZ, as necessary)
• Adoption of the Gregorian calendar
• Astronomical convention (Julian Day Zero, hrs zero....)
• 4004 BC

Please rethink your question.