Write a microwave timer!
Bash + GNU utilities, 33 bytes



dc&lt;&lt;&lt;[6000]sL${1/:/ 60*}+dlLx\&lt;Lp


Try it online!

Or try the test suite.

The input string is passed as an argument, and the output is on stdout.



How it works

First bash expands ${1/:/ 60*} by taking argument 1 and replacing the : with 60* (there's a space before the 60 that StackExchange isn't displaying here).  For example, if the input is 01:30, the expansion is 01 60*30.

Also, \&lt; gets used as the character &lt; without its special meaning in the shell.

So what happens is that dc is run with

[6000]sLminutes60*seconds+dlLx&lt;Lp

as its program (where "minutes" and "seconds" refer to the actual two-digit numbers).

This carries out the following operations:

[6000]               Definition of a macro which pushes 6000 on the stack.
sL                   Save the macro in register L.
minutes 60*seconds+  Compute minutes*60+seconds,
                        which is the total number of seconds.
d                    Duplicate the total number of seconds on the stack.
lLx                  Run macro L, which pushes 6000 on the stack.
                     The stack now looks like:
                        #seconds #seconds 6000
                        (top of stack on the right).
&lt;L                   Pop 6000 and #seconds from the stack;
                        if 6000 &lt; #seconds,
                          then run macro L to push 6000 on the stack again.
                     At this point, the item at the top of the stack is:
                        #seconds, if #seconds &lt;= 6000,
                        6000, if #seconds &gt; 6000.
p                    Print the top of the stack.