A long long time ago, in software development...
There was a VARIANT type, which could hold either of many other datatype values (vartypes), see wikipedia. And one of the vartypes was VT_DATE, aka Variant date, aka DATE (double), aka OLE variant date. It was a datetime value represented as double - 8-byte floating-point number. And, BTW, everything about variant data type is still relevant today, except for the fact that hardcore Windows or COM/ActiveX/OLE programming does not look like mainstream nowadays.
But, sometimes, when you see something like 42842.370277778, chance are that it is variant date. And here is a simple calculator which takes double value, interprets it as variant date and outputs date and time encoded in this double value. Those who curious about format of variant date and history behind that can continue to read after the calculator.
About variant date format.
According to documentation, days are represented by whole number increments starting with 30 December 1899, midnight as time zero. Hour values are expressed as the absolute value of the fractional part of the number.
Yet, there is a catch. Although day values become negative before midnight on December 30, 1899, time-of-day values do not. For example, 6:00 AM is always represented by a fractional value 0.25 regardless of whether the integer representing the day is positive (after December 30, 1899) or negative (before December 30, 1899).
Due to this, value of -1.25 represents 12/29/1899 06:00 AM, and value of -1,30 represents 12/29/1899 07:12 AM. That is, while second double value is less than first double value, as date it is greater than first date.
Also, date values between -1.0 and 0.0 represent the same logical dates as their positive counterparts. That is, -0.5 and 0.5 is the same 12/30/1899 12:00.