Convert milliseconds to TDateTime in Java

Question

I'm in an Android project that requires saving a file with TDateTime type (Delphi). I have my date in milliseconds, but I don't know how to convert milliseconds to TDateTime.

I have something like this:

Date dateInMillis = new Date(System.currentTimeMillis());
double dateInDouble = ???;

I'll be glad for any tips that can help me to resolve this.

Answer 1

Delphi's TDateTime measures time in days. Java follows the Unix standard and measures in milliseconds. To convert between the two you need to scale by the number of milliseconds in a day, 86400000.

The other difference is that the two systems use a different epoch. The Unix epoch, as used by Java, is 00:00, 1 Jan 1970. The Delphi epoch is 00:00, 30 December 1899. The Unix epoch, represented as a Delphi TDateTime is 25569.

So, to convert from milliseconds from the Unix epoch, to days from the Delphi epoch you perform the following calculation:

double delphiDateTime = unixMillis/86400000.0 + 25569.0;