Why doesn't IFERROR work with negative time values?

I created a timestamp grid to track start/stop times and cumulative times for a set of projects. But when I put in the start time without the end time, I get the hashtag string because it's a negative time value. This throws some people off so I tried to use IFERROR, but that doesn't override the hashtag string. Is there any way to leave the cell blank until the finish time is entered?

Because a "string of hashtags" is not a formula error. It is number formatting "error" that simply means Excel cannot display the resulting number using the current number format. The underlying cell value is still a valid number and not an error.

You probably need a more robust number format. Something that will ignore negative values. Try something like "hh:mm;;hh:mm;@" where the first section tells Excel how to display positive values (as time of day), the second (empty) section tells Excel how to display negative values (empty meaning don't display), the third section tells Excel how to display 0 values (as time of day); and the fourth section tells Excel how to display text (@ means to simply display the text).

FWIW, when I was first learning spreadsheets, this was one of my most frequently referenced help files: https://support.microsoft.com/en-us/...7-9c9354dd99f5