just finished writing some code to generate reports by writing data
into an Excel spreadsheet using COM. (Unfortunately XLWT/XLRD etc
wasn't an option because the Excel spreadsheet included charts).

Since there are numerous examples for open, writing to, and saving
Excel files I won't go into that here :-)

One tidbit I found was that Excel time formatted data is stored as a
decimal value between 0.0 and 1.0 depending on how far through the day
it is.

Here is some example code for converting a time to Excel format,
suitable for writing into an Excel time formatted field.

def cnvrt_time(csv_time):
    # csv_time is time in format '00:10:22.634' = 'hh:mm:ss.xxx'
    # excel time format stores time as decimal
    secs_in_day = 24*60*60
    csv_time = csv_time.split('.')[0] # drop milliseconds
    [hrs,mins,secs] = csv_time.split(':')
    total_secs = float(secs)+60*float(mins)+3600*float(hrs)
    time_in_xcl_fmt = total_secs / secs_in_day
    return(time_in_xcl_fmt)

Hope it's useful to somebody else.

Welcome to the group.

One possible source of such tdbits is the xlrd documentation ... e.g.
(1) the general rant about Excel dates at the front of the docs
(2) this convenience function:

xldate_from_time_tuple((hour, minute, second))

     Convert a time tuple (hour, minute, second) to an Excel "date"
value (fraction of a day).