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The easiest way to go about this is to understand that Excel treats a time as a variable of Double type, meaning a number with decimals, for example 42989.375. Here 42989 is today's day (Sept 11, 2017) which started at 00:00 hours. The decimal fraction (0.375) represents the time which has lapsed in this day as Excel counts toward day 42990. Convert to hours by 0.375 * 24 = 9 meaning 9:00am.

Similarly, your 4.6 hours can be converted to a fraction of a day: 4.6 / 24 = 0.196666667 which you can conveniently add to either 0.375 or 42989.375 (depending upon your interest in time only or date and time) to arrive at 0.566666667 which you can convert to hours passed in this day by multiplying with 24 = 13:36

Excel does this conversion in the background following number format you set. Enter = 4/24 in a cell. While the number format is "General" you will see 0.19666667. Change the number format to "Time" and you will see 04:36. Change the format to Date/Time and you will see something else again - predictably, the date and time.

Accordingly, if you have the start time in one cell as 0.375 and a duration of production in another as 0.196666667 you can calculate the completion time by simple addition, as you have in fact tried. The trick is (a) not to be confused by what you see displayed in any of the three cells and (b) not to confuse Excel by what you enter, either. Once you enter 4.6 as a number of hours you must remember to convert this number into a fraction of a day before you can process it as such. On the other hand, if you enter 4:36 (with a colon as separator) Excel will understand this to be a time (depending upon your Regional settings), automatically set a Time Numberformat for the cell where you entered it, and give the cell a value of 0.19666667 which is displayed as 04:36.

You can always get Excel to display the real value of a cell by expressly setting the Numberformat to "General". Mind that, by Excel rules, the "General" format instructs to guess at what you entered and apply a suitable format automatically. So, while a cell's format is "General" to begin with entering a value may change its format. But you can change the automatically set format back to "General" to override Excel's automation.