Based on the function that you posted the following should convert your ticks.

You may want to case statement to make sure the timestamp_id is at least 624511296000000000 to match the function.

SELECT t.Timestamp_id

, DATEADD(

s,

(t.Timestamp_id % CONVERT(BIGINT,864000000000)) / CONVERT(BIGINT,10000000),

DATEADD(

d,

(t.Timestamp_id / CONVERT(BIGINT,864000000000)) - CONVERT(BIGINT,639905),

'17530101'

)

)

FROM (SELECT Timestamp_id FROM (VALUES (624511296000000000),(634956975000000000)) AS Ticks(Timestamp_id)) T

From the look of it the ticks have a datum of year 0.

Given that it appears the 19800101 is the least date that the function will return, you could simplify this further to

SELECT DATEADD(s,

(t.Timestamp_id - 624511296000000000) / 10000000,

'19800101')

FROM (SELECT Timestamp_id FROM (VALUES (624511296000000000),(634956975000000000)) AS Tick(Timestamp_id)) T