Since you posted in a SQL 2005 forum, I'm assuming that you are on that version, which means that the sql 2008 date/time enhancements won't help you:

declare @minutes int = 1056915413

declare @test-2 datetime2 = '01/01/0001'

select DATEADD(minute, @minutes, @test-2)

This code returns:

The six hour part is the GMT offset (which takes it to 2010-07-16 18:53:00.0000000), but this leaves a 15 day discrepancy. I think that most of this is that the day after 9/2/1752 was 9/14/1752, but this only accounts for 11 of those days. The 365.25 accounts for some also... years evenly divisible by 100, unless evenly divisibly by 400, aren't leap years. Which means in 400 years, there are 146097 days (210379680 minutes); your calculation would get 146100 days (210384000 minutes).

So, the conversion you're doing in C# looks correct to me.

You might want to consider a calendar table - list of all dates since 01/01/0001. The following code (on SQL 2008) returns the proper date/time (7/17/2010 00:53:00.0000000) for the minutes:

declare @test-2 datetime2 = '01/01/0001'

-- See Jeff Moden's article

-- The "Numbers" or "Tally" Table: What it is and how it replaces a loop.

-- at http://www.sqlservercentral.com/articles/T-SQL/62867/.

-- NOTE! A permanent tally table will always be MUCH faster

-- than this inline one. See the above article to create your own!

;WITH

Tens (N) AS (SELECT 0 UNION ALL SELECT 0 UNION ALL SELECT 0 UNION ALL

SELECT 0 UNION ALL SELECT 0 UNION ALL SELECT 0 UNION ALL

SELECT 0 UNION ALL SELECT 0 UNION ALL SELECT 0 UNION ALL SELECT 0),

Thousands(N) AS (SELECT 1 FROM Tens t1 CROSS JOIN Tens t2 CROSS JOIN Tens t3),

Millions (N) AS (SELECT 1 FROM Thousands t1 CROSS JOIN Thousands t2),

Tally (N) AS (SELECT ROW_NUMBER() OVER (ORDER BY (SELECT 0)) FROM Millions),

Dates AS (SELECT DateField = DATEADD(day, N-1, @test-2) FROM Tally WHERE N-1 = 1056915413/1440)

SELECT DATEADD(MINUTE, 1056915413 % 1440, DateField)

FROM Dates

Edit: Modified to add the minutes to the date in the calendar table CTE.