Calculate elapsed time using NOW() with conditional formatting - google-sheets

Question

I'm trying to set up a Google Sheet for volunteers at the local dog shelter. The idea is to have an auto-updating spreadsheet that shows at a glance which dogs really need to be walked, in two ways:
conditional formatting the rows with color based on time elapsed:
red if it's been >6 hours since their last walk,
yellow for 3-6 hours, or
green if they were walked <3 hrs ago.
auto-sorting the rows so that the dogs that have gone the longest are always at the top of the list, and when they get walked, they go to the bottom of the list.
Here's where I'm at.
Problem 1: I'm trying to calculate time elapsed by using the NOW() function (which returns the current date and time), minus the time of the last walk. The problem is that if you only enter a time, Google apparently assumes the date is 12/30/1899. So if I put in 8:00 am, and the current time based on NOW() is 4:00 pm, instead of returning 8:00 hours, it calculates the the duration as 10,000,000+ hours that have elapsed since 8:00 am on December 30, 1899 up to the current date and time. (Similar problem discussed here).
If I keep the output cells in the HH:MM format, it'll initially look OK, because it'll just return the HH:MM as 8:00, BUT I can't use that because then the conditional formatting won't work - it's still actually calculating the 10,000,000 hours since 1899. So I can't set duration values for red/green/yellow because all the output values will be slowly increasing every day we get further away from 1899, meaning I would have to reset the ranges daily.
Obviously I could work around this by always including the time and date, but the idea is to have a spreadsheet that is idiot-proof so that any volunteer can use it by just adding the time they walked the dog. It won't work if they have to input the date too.
Problem 2: Assuming I can get the above to work, how can I set it up so the table automatically sorts itself after any change (i.e. when a dog gets walked and the entry gets updated)?
Help?

Answer 1

Please try:
Problem 1
Green is easy, just format all your data that way with standard fill (CF will override this where applicable).
Select A1 and apply a Custom formula is of:
=and($D1<>"",timevalue(now())>$D1+6/24)
with colour Red and Range A:E
Repeat (the order of these two is important):
=and($D1<>"",timevalue(now())>$D1+3/24)
with colour Yellow and Range A:E.
Save rules and close window.
Problem 2
Create a pivot table by selecting ColumnA:E (may have to get rid of some content present low down in the sheet first) and Data > Pivot Table Report..., to Rows Add field Dog name (do not Show totals), to Values Add field Time since last walk and Summarise by:SUM. Name the sheet PT.
In say J2 of your other sheet (not PT) enter:
=query(PT!A:B, "Select * order by B desc ")
May be worth noting that without the day part there might be problems where times span midnight - if you have walkers with insomnia?!

Answer 2

For the autosorting, I found a script elsewhere on stackoverflow and modified it slightly for my purposes:
function onEdit(event){
var sheet = SpreadsheetApp.getActiveSheet();
if(sheet.getName()=='Sheet1'){
var editedCell = sheet.getActiveCell();
var columnToSortBy = 4;
var tableRange = "a2:f91";
if(editedCell.getColumn() == columnToSortBy){
var range = sheet.getRange(tableRange);
range.sort( { column : columnToSortBy, ascending: true } );
}
}
}