If cell A1 has date 4/6/1999, cell A2 has time 7:18:33, and cell A3 is set to "=A1&A2" its value will be 362560.304548611111111. What does that value represent? Is it the number of seconds from January 1,1970, perhaps?
36256 is the number of days since Jan 1, 1900,
and 0.30454861111111 is time value.
You might find these instructive:
Excel is using the "underlying" numeric time value in the concatenation (as in the link). If you did want text, then:
Also, Excel uses some funny dates for the first two months of the year 1900 due to an error in the leap year calculations - so Excel actually starts out a "day behind" so to speak, and is back in order on March 1, 1900. This bug was actually originally in Lotus 1,2,3 but was kept in Excel for compatibility (back in the day, Lotus was used a lot in spreadsheet software).
In theory, it shouldn't be too hard to work it out yourself - just the number of days since Jan 1, 1970 * 86400 I believe (for complete days, anyway) - of course, times and dates are always a tricky thing. You can test your result here: http://www.epochconverter.com/
Since unix uses this time measurement, you can also search for functions and formulas to convert Excel/Excel VBA time to Unix time.