Date to .NET ticks

A .NET tick is 100 nanoseconds and the count starts at 0001-01-01T00:00:00, which is why the numbers are so large: 2024 is around 638 quadrillion ticks. The unusual epoch is a deliberate design choice — starting at year one means every representable date is a positive number, so DateTime never needs a sign and comparisons are simple integer comparisons.

The conversion people usually want is to or from Unix time, and the offset is a fixed constant: 621,355,968,000,000,000 ticks between 0001-01-01 and 1970-01-01. So Unix seconds are (ticks - 621355968000000000) / 10000000. That constant is worth recognising — if a number looks like a tick value and subtracting it gives something sensible, you have found a .NET timestamp.

One thing ticks do not carry is a time zone. A DateTime has a separate Kind property saying whether it is UTC, local or unspecified, and the ticks are the same number regardless — which is the source of a great deal of confusion when a value crosses a process boundary and the Kind is lost. DateTimeOffset exists precisely because of that, and storing one is almost always the better choice.